ARM: AM43XX: hwmod: Add rtc hwmod

The patch registers the rtc hwmod on AM437x chips.  The RTC module is
physically present on the AM438x SoC used on AM43X-EPOS-EVM, but it is
permanently disabled. A secure RTC is used instead on these devices,
where needed. Hence adding it selectively using a separate list to get
RTC Module functional on the other am43x SoCs used on am437x-gp-evm
and am437x-sk-evm.

Signed-off-by: Keerthy <j-keerthy@ti.com>
[paul@pwsan.com: cleaned up patch description]
Signed-off-by: Paul Walmsley <paul@pwsan.com>