riscv: dts: Move BUILTIN_DTB_SOURCE to common Kconfig

riscv: dts: Move BUILTIN_DTB_SOURCE to common Kconfig

The BUILTIN_DTB_SOURCE was only configured for K210 before. Since
SOC_BUILTIN_DTB_DECLARE was removed at commit d5805af9fe9f ("riscv: Fix
builtin DTB handling") from patch [1], the kernel cannot choose one of the
dtbs from then on and always take the first one dtb to use. Then, another
commit 0ddd7eaffa64 ("riscv: Fix BUILTIN_DTB for sifive and microchip soc")
from patch [2] supports BUILTIN_DTB_SOURCE for other SoCs. However, this
feature will only work if the Kconfig we use links the dtb we expected in
the first place as mentioned in the thread [3]. Thus, a config
BUILTIN_DTB_SOURCE is needed for all SoCs to choose one dtb to use.

For some considerations, this patch also removes default y if XIP_KERNEL
for BUILTIN_DTB, as this requires setting a proper dtb to use on the
BUILTIN_DTB_SOURCE, else the kernel with XIP but does not set
BUILTIN_DTB_SOURCE or unselect BUILTIN_DTB will not boot.

Also, this patch removes the default dtb string for k210 from Kconfig to
nommu_k210_defconfig and nommu_k210_sdcard_defconfig to avoid complex
Kconfig settings for other SoCs in the future.

[1] https://lore.kernel.org/linux-riscv/20201208073355.40828-5-damien.lemoal@wdc.com/
[2] https://lore.kernel.org/linux-riscv/20210604120639.1447869-1-alex@ghiti.fr/
[3] https://lore.kernel.org/linux-riscv/CAK7LNATt_56mO2Le4v4EnPnAfd3gC8S_Sm5-GCsfa=qXy=8Lrg@mail.gmail.com/

Signed-off-by: Yangyu Chen <cyy@cyyself.name>
Reviewed-by: Conor Dooley <conor.dooley@microchip.com>
Acked-by: Palmer Dabbelt <palmer@rivosinc.com>
Signed-off-by: Conor Dooley <conor.dooley@microchip.com>