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>

arch/arm/mach-at91/pm_slowclock.S

@@ -70,6 +70,8 @@ tmp2	.req	r5
 
 	.text
 
+	.arm
+
 /* void at91_slow_clock(void __iomem *pmc, void __iomem *sdramc,
  *			void __iomem *ramc1, int memctrl)
  */