spi: dw-mmio: add MSCC Jaguar2 support

Unfortunately, the Jaguar2 CPU_SYSTEM_CTRL register set has a different
layout than the Ocelot one. Handle that while keeping most of the code
common.

Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Reviewed-by: Gregory CLEMENT <gregory.clement@bootlin.com>
Signed-off-by: Mark Brown <broonie@kernel.org>