flash/nor/rp2040: flash bank target switching for RP2350

flash/nor/rp2040: flash bank target switching for RP2350

RP2350 can switch either core to Cortex-M33 or RISC-V.
The different architectures have to be supported as
distinct targets in OpenOCD.

Introduce 'rp2xxx _switch target' Tcl command to adapt flash
bank to architecture changes.

Keep the target and priv pointers intact until a flash operation
is finished to prevent sudden change in the middle of write/erase.

Signed-off-by: Tomas Vanek <vanekt@fbl.cz>
Change-Id: I764354ab469e253042128958dfe70c09d04d6411
Reviewed-on: https://review.openocd.org/c/openocd/+/8448
Tested-by: jenkins
Reviewed-by: Antonio Borneo <borneo.antonio@gmail.com>