mfd: mt6397: Use named IRQs instead of index (keys)

Some pmics of the mt6397 family (such as MT6358), have two IRQs per
physical key: one for press event, another for release event.

The mtk-pmic-keys driver assumes that each key only has one
IRQ. The key index and the RES_IRQ resource index have a 1/1 mapping.

This won't work for MT6358, as we have multiple resources (2) for one key.

To prepare mtk-pmic-keys to support MT6358, retrieve IRQs by name
instead of by index.

Note: The keys_resources are not part of the device-tree bindings so
this won't break any DT schemas.

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