drivers: uart_mcux_flexcomm: Enable as wakeup src

Add the feature for the flexcomm uart to be able to be a wakeup source
from low power modes. To be able to do this on a relevant platform, the
DT node for the UART needs to have the wakeup-source property and define
a "sleep" clock. The details of handling the sleep clock and default
clock are still platform specific but handled by the clock control
driver of the platform, so that this code should be able to be
applicable to any platform that follows the same DT definition.

Signed-off-by: Declan Snyder <declan.snyder@nxp.com>