Implement the exec_mipi_pmic_seq_element callback for the CHT Whiskey Cove
PMIC.

On some CHT devices this fixes the LCD panel not lighting up when it was
not initialized by the GOP, because an external monitor was plugged in and
the GOP initialized only the external monitor.
Reviewed-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190107111556.4510-3-hdegoede@redhat.com