ARM: omap4: Provide do_wfi() for Thumb-2

For CONFIG_THUMB2_KERNEL, the existing definition of do_wfi() will
insert invalid code into the instruction stream.

Any assembler which can assemble Thumb-2 is guaranteed to accept
the "wfi" mnemonic, so for the Thumb-2 case, just use the mnemonic.

The ARM case is left as-is.

Signed-off-by: Dave Martin <dave.martin@linaro.org>
Signed-off-by: Kevin Hilman <khilman@ti.com>