ARM: at91: fix pm_suspend.S compilation when ARMv6 is selected

When compiling for multiplatform for both ARMv6 and ARMv7, the default compiler
flags are for ARMv6, and results in:

arch/arm/mach-at91/pm_suspend.S:144: Error: selected processor does not support ARM mode `dsb'

Enforce ARMv7 flags for pm_suspend.o when CPU_V7 is selected.

Reported-by: kbuild test robot <fengguang.wu@intel.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>