drivers: flash: sam0: Fix incorrect page write size

According to the datasheet, one page is made of 64 bytes
and one row is made of 4 pages, that is 256 bytes.
This MR fixes the incorrect page write size for sam0
and sets it to 64 bytes.

Signed-off-by: Taras Zaporozhets <zaporozhets.taras@gmail.com>