ARM: at91/pm_slowclock: function slow_clock() accepts parameters

Change slow_clock()/at91_slow_clock() prototype to accept the PMC
base address and one or two RAM controller addresses by parameters.
The r0, r1 and r2 registers are used differently and preserved during
function call.
Those values are defined in pm.c and slow_clock() function is called
from there with its new parameters.

This will allow to have a soc independent pm_slowclock.

Signed-off-by: Jean-Christophe PLAGNIOL-VILLARD <plagnioj@jcrosoft.com>
Ached-by: Nicolas Ferre <nicolas.ferre@atmel.com>