driver: espi: npcx: add the controller attached flash sharing API

Add the flash channel support by implementing the controller attached
flash sharing APIs, including flash_read, flash_write, and flash_erase.
For flash_read, the max data payload the module can support is 64 bytes
in one transaction.
For flash_write, the max data payload the module can support is 16
bytes in one transaction.

Signed-off-by: Mulin Chao <mlchao@nuvoton.com>
Signed-off-by: Jun Lin <CHLin56@nuvoton.com>