arch: arm: nxp: Fixup HAS_MCUX_RTC

When the RTC support get added we had a select on HAS_RTC, however this
Kconfig symbol didn't exist.  Clean this up to match the pattern of
HAS_MCUX_RTC.  The driver now depends on that and the SoC selects it.

Signed-off-by: Kumar Gala <kumar.gala@linaro.org>