drivers: flash: Initial support flash driver on EK-RA8M1

Initial commit for flash driver support on board using RA8 MCUs
* drivers: flash: implementation for flash driver on EK-RA8M1
* dts: arm: add device node for flash of EK-RA8M1
* boards: arm: enable support flash driver for ek_ra8m1, update
board documentation

Signed-off-by: Quy Tran <quy.tran.pz@renesas.com>
Signed-off-by: Duy Phuong Hoang. Nguyen <duy.nguyen.xa@renesas.com>