dts: arm: atmel: samx7xx21: fix flash erase-blocks

The flash erase-blocks property was a bit malformed, it has 254 pages
of size 8192 after the first 8 pages of size 2048 to reach the total
size of 2048K. This caused issues when trying to erase the last pages
of the flash, which caused the flash erase_blocks test suite to fail.

Adjust the erase-blocks to match the flash layout.

Signed-off-by: Bjarki Arge Andreasen <bjarki.andreasen@nordicsemi.no>