rtc: Group Kconfig entries by vendor

The RTC entries are mostly grouped by vendor. Move the few outliers in
place.
Also, change the one occurrence of 'nxp' to 'NXP' to make all NXP
entries consistent.

Signed-off-by: Soren Brinkmann <soren.brinkmann@xilinx.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>