drivers: mcux_rtc: convert to DT_INST defines

Convert driver to use DT_INST_ defines.  Replace dts_fixup.h use
for DT_RTC_0_NAME with DT_INST_0_NXP_KINETIS_RTC_LABEL to be
consistent.  Also, remove the aliases that had been used for this
driver in various nxp_k*.dtsi.

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