rtc: generic: allow building on all architectures

There are four architectures using this driver, but since we can
build it with COMPILE_TEST, we should try dealing with the absence
of the asm/rtc.h header file, to avoid getting a build error:

drivers/rtc/rtc-generic.c:12:21: fatal error: asm/rtc.h: No such file or directory

This creates an alternative use of the driver, allowing architectures
to pass a set of rtc_class_ops in platform data. We can convert the
four architectures to use this and then remove the original
code.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>