arm: omap3: beagle: Ensure msecure is mux'd to be able to set the RTC

Without msecure beeing high it isn't possible to set (or start)
the RTC.

Tested with a BeagleBoard C4.

Signed-off-by: Alexander Holler <holler@ahsoftware.de>
Signed-off-by: Tony Lindgren <tony@atomide.com>