regmap: irq: Fix possible ZERO_SIZE_PTR pointer dereferencing error.

Since we cannot make sure the 'chip->num_regs' will always be none zero
from the users, and then if 'chip->num_regs' equals to zero by mistake
or other reasons, the kzalloc() will return ZERO_SIZE_PTR, which equals
to ((void *)16).

So this patch fix this with just checking the 'chip->num_regs' before
calling kzalloc().

This also sorts the header files in alphabetical order at the same time.

Signed-off-by: Xiubo Li <Li.Xiubo@freescale.com>
Signed-off-by: Mark Brown <broonie@linaro.org>