rtc: at91rm9200: move register definitions to C file

The header was simply moved from the arm mach folder to drivers/rtc but
there is not point in having it separated from the driver.

Also remove unused bit definitions and use BIT and GENMASK.

Link: https://lore.kernel.org/r/20191229204421.337612-4-alexandre.belloni@bootlin.com


Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>