flash: sam0: set write_block_size=1 when EMULATE_BYTE_PAGES is set

The sam0 has a page size of 64 bytes, making it incompatible with
the nvs driver that specifies a maximum write block size of
32 bytes. When CONFIG_SOC_FLASH_SAM0_EMULATE_BYTE_PAGES is set,
it should report a write block size of 1 byte, which is compatible
with nvs.

Signed-off-by: Adam Serbinski <aserbinski@gmail.com>