mfd: mt6397: Add PMIC keys for MT6358

 Add compatible strings and interrupts for pmic keys
which serves as child device of MFD.

MT6358 has two interrupts per key: one for press, another one for
release (_R)

Signed-off-by: Mattijs Korpershoek <mkorpershoek@baylibre.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>