arm, am33x: make RTC32K OSC enable configurable

As
http://www.denx.de/wiki/view/U-Boot/DesignPrinciples#2_Keep_it_Fast


states:
"Initialize devices only when they are needed within U-Boot"

enable the RTC32K OSC only, if CONFIG_SPL_AM33XX_ENABLE_RTC32K_OSC is
enabled. Enable this in ti_am335x_common.h, so all boards in mainline
should work as before.

Signed-off-by: Heiko Schocher <hs@denx.de>
Cc: Tom Rini <trini@ti.com>