spi: dw-apb-ssi: Add compatible string for DesignWare DWC_ssi

spi: dw-apb-ssi: Add compatible string for DesignWare DWC_ssi

This patch adds compatible string "snps,dwc-ssi-1.01a" to the above DT
binding document, to provide support for DesignWare DWC_ssi IP [1].

Current driver supports DW_apb_ssi IP [2].

References:
[1] https://www.synopsys.com/dw/ipdir.php?c=dwc_ssi
[2] https://www.synopsys.com/dw/ipdir.php?c=DW_apb_ssi

Signed-off-by: Wan Ahmad Zainie <wan.ahmad.zainie.wan.mohamad@intel.com>
Link: https://lore.kernel.org/r/20200505130618.554-5-wan.ahmad.zainie.wan.mohamad@intel.com
Signed-off-by: Mark Brown <broonie@kernel.org>