ARM: OMAP Add RTC support to the (new style) Menelaus driver.

This involved several bugfixes and cleanups to the IRQ handling logic.
Code for the second register bank never worked, and even clobbered
non-RTC registers; and the handler enabled IRQs for interrupts that
should have stayed masked...

Passes 'rtctest' on H4, other than lack of "periodic" (1/N second) IRQs.
System wakeup alarms (from suspend-to-RAM) work too.

The battery keeps the RTC active over power off, so once you set clock
(rdate/ntpdate/etc, then "hwclock -w") then RTC_HCTOSYS at boot time
will behave as expected.

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Tony Lindgren <tony@atomide.com>