ARM: at91: pm_slowclock: fix the compilation error

When compiling the kernel in thumb2 (CONFIG_THUMB2_KERNEL option activated), we
hit a compilation crash. The error message is listed below:

---8< -----
Error: cannot use register index with PC-relative addressing -- `str r0,.saved_lpr'
--->8----

Add the .arm directive in the assembly files related to power management.

Signed-off-by: Wenyou Yang <wenyou.yang@atmel.com>
Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>