hw/hppa/machine: Allow up to 3840 MB total memory

hw/hppa/machine: Allow up to 3840 MB total memory

The physical hardware allows DIMMs of 4 MB size and above, allowing up
to 3840 MB of memory, but is restricted by setup code to 3 GB.
Increase the limit to allow up to the maximum amount of memory.

Btw. the memory area from 0xf000.0000 to 0xffff.ffff is reserved by
the architecture for firmware and I/O memory and can not be used for
standard memory.

An upcoming 64-bit SeaBIOS-hppa firmware will allow more than 3.75GB
on 64-bit HPPA64. In this case the ram_max for the pa20 case will change.

Signed-off-by: Helge Deller <deller@gmx.de>
Noticed-by: Nelson H. F. Beebe <beebe@math.utah.edu>
Fixes: b7746b1194c8 ("hw/hppa/machine: Restrict the total memory size to 3GB")
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Tested-by: Bruno Haible <bruno@clisp.org>