linker: make sure the macro arguments both point to the vma

If the rom region can be accessed via a virtual memory address,
then __rodata_region_end will point to the vma, while __rom_region_start
is the lma. This patch makes sure the two values are in the vma address
space.

This is in prepartion for a future patch which will allow the rom to be
accessed via a virtual memory address.

Signed-off-by: Andriy Gelman <andriy.gelman@gmail.com>