rtc: stmp3xxx: Port stmp-functions to mxs-equivalents

The stmp3xxx  driver used to include functions from a stmp-specific
include. Because of consolidation, plat-stmp has now been removed and
merged with the compatible mach-mxs.

Use the apropriate mxs-functions for transition. The accessors will be
converted to readl/writel in a later patch.

Signed-off-by: Wolfram Sang <w.sang@pengutronix.de>
Tested-by: Shawn Guo <shawn.guo@freescale.com>
Signed-off-by: John Stultz <john.stultz@linaro.org>