Place the key bit in struct mtk_pmic_keys_regs to enhance this
driver's flexibility, in preparation for adding support for more
PMICs.

While at it, remove the definition of MTK_PMIC_RST_KEY_MASK as
we are now dynamically setting the keymask relatively to the keys
that are defined in the newly added rst_en_mask variable, on a
per-key basis.

This commit brings no functional changes.
Signed-off-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Reviewed-by: Mattijs Korpershoek <mkorpershoek@baylibre.com>
Link: https://lore.kernel.org/r/20220524093505.85438-2-angelogioacchino.delregno@collabora.comSigned-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>