arm, am33xx: move rtc32k_enable() to common place

move rtc32k_enable() to common place so all am33xx boards can use it.

Signed-off-by: Heiko Schocher <hs@denx.de>
Cc: Matt Porter <mporter@ti.com>
Cc: Lars Poeschel <poeschel@lemonage.de>
Cc: Tom Rini <trini@ti.com>
Cc: Enric Balletbo i Serra <eballetbo@iseebcn.com>