flash/nor/rp2040: preparatory refactoring

flash/nor/rp2040: preparatory refactoring

Prepend stack_grab_and_prep() function name by rp2040_ prefix.

Introduce target helper variable in rp2040_stack_grab_and_prep()
and use it instead of dereferencing bank->target several times.

Move flash ID reading code to the new rp2040_spi_read_flash_id()
function.

Change-Id: I9d6e51e17e36e6230155a586065499f2f260089a
Signed-off-by: Tomas Vanek <vanekt@fbl.cz>
Reviewed-on: https://review.openocd.org/c/openocd/+/7185
Tested-by: jenkins
Reviewed-by: Antonio Borneo <borneo.antonio@gmail.com>