mfd: pm8xxx: add support to pm8821

This patch adds support to PM8821 PMIC and interrupt support.
PM8821 is companion device that supplements primary PMIC PM8921 IC.

Signed-off-by: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>