drivers: counter: Introduce mcux rtc shim

Adds a shim layer around the mcux rtc driver to adapt it to the zephyr
counter interface. Portions of this driver are reused from the existing
rtc driver in drivers/rtc/rtc_mcux.c.

The hardware supports a single alarm only and a fixed wrap value.

Signed-off-by: Maureen Helm <maureen.helm@nxp.com>