riscv: linker: align end of ROM area to a "page" boundary for QEMU

QEMU does its dynamic instruction translation in 4096-byte-sized chunks.
Therefore it doesn't vet PMP access for each instruction prefetch but
for the whole "page" at once. If the end of the ROM area lands in the
middle of such a page with a corresponding PMP entry then any instruction
access within that page will fault even if it is located in the PMP
mapped portion.

To work around this QEMU peculiarity, we simply align the end of the ROM
area to a page boundary.

Signed-off-by: Nicolas Pitre <npitre@baylibre.com>