board: rock5b-rk3588: add memory gaps into kernel's DTB

RK3588 has two memory gaps when using 16 GiB DRAM size:
[0x3fc000000 , 0x3fc500000]
and
[0x3fff00000 , 0x3ffffffff]

If the kernel is agnostic to these gaps, accessing the area causes
a SError panic.

Hence, add reserved memory areas in kernel's DTB before booting.

Signed-off-by: Eugen Hristev <eugen.hristev@collabora.com>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>