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>
diff --git a/README b/README
index 39b3fe6..f0ffaf4 100644
--- a/README
+++ b/README
@@ -4319,6 +4319,9 @@
 
 		NOTE : currently only supported on AM335x platforms.
 
+- CONFIG_SPL_AM33XX_ENABLE_RTC32K_OSC:
+		Enables the RTC32K OSC on AM33xx based plattforms
+
 Freescale QE/FMAN Firmware Support:
 -----------------------------------