avr32: cleanup - use _AC macro to define PAGE_SIZE

PAGE_SIZE is used both from assembly and C code. We want to have type
specifiers when using it from C, but this will make the assembler
confused, so we need to make it conditional.

This is exactly what the _AC macro is for, so using it allows us to
get rid of a few lines of cpp noise.

Signed-off-by: Cyrill Gorcunov <gorcunov@gmail.com>
Signed-off-by: Haavard Skinnemoen <hskinnemoen@atmel.com>