ARM: at91/pm_slowclock: rename register to named define

This patch will give a name to ARM registers in the assembly
source code. It is done to simplify the code reading and
the passing of parameters to functions.

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