csky: Use top-down mmap layout

csky: Use top-down mmap layout

Follow riscv mmap layout with commit "riscv: make mmap allocation
top-down by default (54c95a11cc1b)".

Before:
 cat /proc/self/maps
00008000-000dc000 r-xp 00000000 fe:00 17         /bin/busybox
000dc000-000dd000 r--p 000d3000 fe:00 17         /bin/busybox
000dd000-000de000 rw-p 000d4000 fe:00 17         /bin/busybox
2aaa8000-2aac6000 r-xp 00000000 fe:00 196        /lib/ld-2.28.9000.so
2aac6000-2aac7000 r-xp 00000000 00:00 0          [vdso]
2aac7000-2aac8000 r--p 0001e000 fe:00 196        /lib/ld-2.28.9000.so
2aac8000-2aac9000 rw-p 0001f000 fe:00 196        /lib/ld-2.28.9000.so
2aac9000-2aad9000 r-xp 00000000 fe:00 219        /lib/libresolv-2.28.9000.so
2aad9000-2aada000 r--p 0000f000 fe:00 219        /lib/libresolv-2.28.9000.so
2aada000-2aadb000 rw-p 00010000 fe:00 219        /lib/libresolv-2.28.9000.so
2aadb000-2aadd000 rw-p 00000000 00:00 0
2aadd000-2ac27000 r-xp 00000000 fe:00 203        /lib/libc-2.28.9000.so
2ac27000-2ac28000 ---p 0014a000 fe:00 203        /lib/libc-2.28.9000.so
2ac28000-2ac2a000 r--p 0014a000 fe:00 203        /lib/libc-2.28.9000.so
2ac2a000-2ac2b000 rw-p 0014c000 fe:00 203        /lib/libc-2.28.9000.so
2ac2b000-2ac2e000 rw-p 00000000 00:00 0
7fb99000-7fbba000 rwxp 00000000 00:00 0          [stack]

After:
 cat /proc/self/maps
00008000-000dc000 r-xp 00000000 fe:00 17         /bin/busybox
000dc000-000dd000 r--p 000d3000 fe:00 17         /bin/busybox
000dd000-000de000 rw-p 000d4000 fe:00 17         /bin/busybox
77e13000-77f5d000 r-xp 00000000 fe:00 203        /lib/libc-2.28.9000.so
77f5d000-77f5e000 ---p 0014a000 fe:00 203        /lib/libc-2.28.9000.so
77f5e000-77f60000 r--p 0014a000 fe:00 203        /lib/libc-2.28.9000.so
77f60000-77f61000 rw-p 0014c000 fe:00 203        /lib/libc-2.28.9000.so
77f61000-77f66000 rw-p 00000000 00:00 0
77f66000-77f76000 r-xp 00000000 fe:00 219        /lib/libresolv-2.28.9000.so
77f76000-77f77000 r--p 0000f000 fe:00 219        /lib/libresolv-2.28.9000.so
77f77000-77f78000 rw-p 00010000 fe:00 219        /lib/libresolv-2.28.9000.so
77f78000-77f96000 r-xp 00000000 fe:00 196        /lib/ld-2.28.9000.so
77f96000-77f97000 r-xp 00000000 00:00 0          [vdso]
77f97000-77f98000 r--p 0001e000 fe:00 196        /lib/ld-2.28.9000.so
77f98000-77f99000 rw-p 0001f000 fe:00 196        /lib/ld-2.28.9000.so
7fd7b000-7fd9c000 rwxp 00000000 00:00 0          [stack]

Signed-off-by: Guo Ren <guoren@linux.alibaba.com>
Cc: Arnd Bergmann <arnd@arndb.de>