board: enable 32kHz RTC OSC at B&R boards

Since RTC-Clock is needed on all B&R boards, the OSC will be enabled
wihtin SPL-stage.

Signed-off-by: Hannes Petermaier <oe5hpm@oevsv.at>

include/configs/bur_am335x_common.h

@@ -22,7 +22,7 @@
 /* Timer information */
 #define CONFIG_SYS_PTV			2	/* Divisor: 2^(PTV+1) => 8 */
 #define CONFIG_SYS_TIMERBASE		0x48040000	/* Use Timer2 */
-
+#define CONFIG_SPL_AM33XX_ENABLE_RTC32K_OSC	/* enable 32kHz OSC at bootime */
 #define CONFIG_SPL_POWER_SUPPORT
 #define CONFIG_POWER_TPS65217