[ARM] 4294/1: ns9xxx: Determine system clock from PLL register settings

The function attribute const is abused here as the PLL register is read.  But I
think this is all right because the PLL register cannot change without a reset.

Note: This patch depends on 4293/1

Signed-off-by: Uwe Kleine-König <ukleinek@informatik.uni-freiburg.de>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>