ARM: cpu: at91: clean up peripheral clock code

ARM: cpu: at91: clean up peripheral clock code

Due to introducing the new peripheral clock handle functions,
use these functions to clean up the duplicated code.

Meanwhile, remove unneeded header file include, at91_pmc.h.

Signed-off-by: Wenyou Yang <wenyou.yang@atmel.com>
Tested-by: Heiko Schocher <hs@denx.de>
Reviewed-by: Andreas Bießmann <andreas.devel@googlemail.com>
[fixup for arm920t code]
Signed-off-by: Andreas Bießmann <andreas.devel@googlemail.com>