rtc: omap: prevent disabling of clock/module during suspend

If RTC is running from an internal clock source, the RTC module can't
be disabled; otherwise it stops ticking completely. Current suspend
handler implementation disables the clock/module unconditionally,
instead fix this by disabling the clock only if we are running on
external clock source, which is not affected by suspend.

The prevention of disabling the clock must be done via implementing
the runtime_pm handlers for the device, and returning an error code
from the runtime suspend handler; otherwise OMAP core PM will disable
the clocks for the driver.

Signed-off-by: Tero Kristo <t-kristo@ti.com>
Signed-off-by: Keerthy <j-keerthy@ti.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>