linker: nios2: align rodata section

During testing with sorting section by alignment with qemu_nios2,
if rodata section is not aligned on 4-byte boundary and its size
not of multiple of 4, it would never boot correctly. So align
the rodata here. This is in preparation to enable the linker
option to sort sections by alignment.

Signed-off-by: Daniel Leung <daniel.leung@intel.com>