drivers: flash: rpi_pico: add support for rp2350 flash controller

The Raspberry Pi Pico 2 uses a QMI flash controller, which differs from the
SSI controller used in the original Pico. Zephyr already has support for
the SSI controller, but lacked support for QMI.

This change adds the QMI flash controller implementation in the
flash_rpi_pico.c driver file. Additionally, the RP2350 SoC devicetree file
(rp2350.dtsi) has been updated to enable and describe the flash controller
for Pico 2.

Signed-off-by: Hanan Arshad <hananarshad619@gmail.com>