board: atmel: siemens: clean up PLLB code

Due to introducing the new PLLB clock handle functions,
use these functions to clean up the PLLB enable code.

Signed-off-by: Wenyou Yang <wenyou.yang@atmel.com>
Reviewed-by: Andreas Bießmann <andreas.devel@googlemail.com>
Tested-by: Heiko Schocher <hs@denx.de>