arch: common: fix copy for ramfunc region during XIP init

ramfunc region is copied into RAM from FLASH region during XIP init. We
copy from the loadaddr of the region, and were previously loading to the
symbol __ramfunc_start. This is incorrect when using an MPU with
alignment requirements, as the __ramfunc_start symbol may have padding
placed before it in the region. The __ramfunc_start symbol still needs
to be aligned in order to be used by the MPU though, so define a new
symbol __ramfunc_region_start, and use that symbol when copying the
__ramfunc region from FLASH to RAM.

Fixes #75296

Signed-off-by: Daniel DeGrasse <daniel.degrasse@nxp.com>