spi: realtek-rtl: Add support for Realtek RTL838x/RTL839x SPI controllers

spi: realtek-rtl: Add support for Realtek RTL838x/RTL839x SPI controllers

This driver likely also supports earlier (RTL8196) and later (RTL93xx)
SoCs.

The SPI hardware in these SoCs is specifically intended for connecting NOR
bootflash chips, and only used for that in dozens of examined devices.
However boiled down to basics, it's really just a half-duplex SPI
controller.

The hardware appears to have a vestigial second chip-select control, but
it hasn't been seen in the wild and is thus not supported.

Signed-off-by: Bert Vermeulen <bert@biot.com>
Link: https://lore.kernel.org/r/20210120135928.246054-3-bert@biot.com
Signed-off-by: Mark Brown <broonie@kernel.org>