ACPI / PMIC: Add support for executing PMIC MIPI sequence elements

DSI LCD panels describe an initialization sequence in the Video BIOS
Tables using so called MIPI sequences. One possible element in these
sequences is a PMIC specific element of 15 bytes.

Although this is not really an ACPI opregion, the ACPI opregion code is the
closest thing we have. We need to have support for these PMIC specific MIPI
sequence elements somwhere. Since we already instantiate a special platform
device for Intel PMICs for the ACPI PMIC OpRegion handler to bind to,
with PMIC specific implementations of the OpRegion, the handling of MIPI
sequence PMIC elements fits very well in the ACPI PMIC OpRegion code.

This commit adds a new intel_soc_pmic_exec_mipi_pmic_seq_element()
function, which is to be backed by a PMIC specific
exec_mipi_pmic_seq_element callback. This function will be called by the
i915 code to execture MIPI sequence PMIC elements.

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/20190107111556.4510-2-hdegoede@redhat.com