riscv: drop the use of XIP_OFFSET in XIP_FIXUP_OFFSET

riscv: drop the use of XIP_OFFSET in XIP_FIXUP_OFFSET

XIP_OFFSET is the hard-coded offset of writable data section within the
kernel.

By hard-coding this value, the read-only section of the kernel (which is
placed before the writable data section) is restricted in size.

As a preparation to remove this hard-coded macro XIP_OFFSET entirely, stop
using XIP_OFFSET in XIP_FIXUP_OFFSET. Instead, use CONFIG_PHYS_RAM_BASE and
_sdata to do the same thing.

While at it, also add a description for XIP_FIXUP_OFFSET.

Signed-off-by: Nam Cao <namcao@linutronix.de>
Reviewed-by: Alexandre Ghiti <alexghiti@rivosinc.com>
Link: https://lore.kernel.org/r/dba0409518b14ee83b346e099b1f7f934daf7b74.1717789719.git.namcao@linutronix.de
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>