ARM: at91: armv7: clean up UTMI PLL handle code

Due to introducing the UTMI PLL enable function, use this function
to reduce the duplicated code.

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