drivers: flash: add andes qspi xip flash driver

Add a flash driver that is used to perform flash operations on a flash
chip that is connected to an Andes QSPI controller and is used for XIP
mode.

The driver is as small as possible, because necessary code has to be
placed in RAM. It is not possible to fetch code from flash while
performing erase/write operations.

Signed-off-by: Dawid Niedzwiecki <dawidn@google.com>