spi: microchip: pci1xxxx: Add driver for SPI controller of PCI1XXXX PCIe switch

Microchip pci1xxxx is a PCIe switch with a multi-function endpoint on one of its
downstream ports. SPI is one of the functions in the multi-function endpoint. This
function has 2 SPI masters, operates at a maximum frequency of 30 MHz and supports
7 client devices per master. This patch adds complete functionality to the SPI
function except for suspend and resume.

Signed-off-by: Tharun Kumar P <tharunkumar.pasumarthi@microchip.com>
Link: https://lore.kernel.org/r/20221006050514.115564-2-tharunkumar.pasumarthi@microchip.com
Signed-off-by: Mark Brown <broonie@kernel.org>