drivers: memc: memc_mcux_flexspi: support diff RX clock source on port B

Some instances of the FlexSPI IP support a different clock source being
used for port B of the FlexSPI instance. Add a devicetree property and
driver support to enable configuring this property of the hardware.

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