Modern processors may need to drain the WB before WFI

Since WFI may cause the processor to enter a low-power mode, data may
still be in the write buffer. This patch adds a DSB (or DWB) to the
cpu_(v6|v7)_do_idle functions before the WFI.

Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>