dt-bindings: spi: mxic: The interrupt property is not mandatory

The interrupt property is not mandatory at all, this property should not
be part of the required properties list, so move it into the optional
properties list.

Fixes: 326e5c8d ("dt-binding: spi: Document Macronix controller bindings")
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Reviewed-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/linux-mtd/20211216111654.238086-8-miquel.raynal@bootlin.com

Documentation/devicetree/bindings/spi/spi-mxic.txt

@@ -8,11 +8,13 @@ Required properties:
 - reg: should contain 2 entries, one for the registers and one for the direct
        mapping area
 - reg-names: should contain "regs" and "dirmap"
-- interrupts: interrupt line connected to the SPI controller
 - clock-names: should contain "ps_clk", "send_clk" and "send_dly_clk"
 - clocks: should contain 3 entries for the "ps_clk", "send_clk" and
 	  "send_dly_clk" clocks
 
+Optional properties:
+- interrupts: interrupt line connected to the SPI controller
+
 Example:
 
 	spi@43c30000 {