soc: arm: nxp: use zephyr,chosen flash node for RT6xx flash base address

Update rt6xx base address calculation to use the zephyr,chosen flash node
to determine flash base address. Note that due to the external flash
controller, the flexSPI base address must be used when the flash device
is on the flexSPI bus.

Signed-off-by: Daniel DeGrasse <daniel.degrasse@nxp.com>