drivers/flash: it51xxx: Add the M1K flash driver

The flash M1K driver supports read (up to 1K), write (1K), and
erase (4K) operations, which can be accessed via DLM.
Accessible flash regions include internal e-Flash or external SPI
flash via FSCE# or FSCE1#.

Signed-off-by: Tim Lin <tim2.lin@ite.corp-partner.google.com>