spi: dw-mmio: allow multiple instances

Use the platform_device id to uniquely identify each SPI master instance.

Cc: Jean-Hugues Deschenes <jean-hugues.deschenes@octasic.com>
Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Mark Brown <broonie@linaro.org>