mfd: lpc_ich: Add support for Intel Gemini Lake SoC

Like Intel Apollo Lake, Gemini Lake exposes the serial SPI flash device BAR
through hidden P2SB PCI device. We use the same mechanism than Apollo Lake
to read the BAR and pass it to the driver.

Signed-off-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>