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/include/configs/ti_am335x_common.h b/include/configs/ti_am335x_common.h
index 10fe47f..0f6fa62 100644
--- a/include/configs/ti_am335x_common.h
+++ b/include/configs/ti_am335x_common.h
@@ -18,6 +18,7 @@
 #define CONFIG_SYS_CACHELINE_SIZE       64
 #define CONFIG_MAX_RAM_BANK_SIZE	(1024 << 20)	/* 1GB */
 #define CONFIG_SYS_TIMERBASE		0x48040000	/* Use Timer2 */
+#define CONFIG_SPL_AM33XX_ENABLE_RTC32K_OSC
 
 #include <asm/arch/omap.h>