drivers: flash: add flash driver support for Atmel SAM E70

This patch adds a flash driver for the Atmel SAM E70 SoC. The driver has
been kept simple by considering that the flash is only composed of 8-KiB
blocks. Indeed an area at the beginning of the flash might be erased
with a smaller granularity, and the other blocks can also be erased with
a higher granularity. It also only handles the global read/write
protection, not the 128-KiB lock regions. A write error is returned if
a region is locked.

Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>