sandbox: Rename 'num-gpios' property to avoid dtc warning

At present dtc produces these warnings when compiling sandbox:

arch/sandbox/dts/test.dtb: Warning (gpios_property):
	Could not get phandle node for /base-gpios:num-gpios(cell 0)
arch/sandbox/dts/test.dtb: Warning (gpios_property):
	Missing property '#gpio-cells' in node /reset-ctl or bad phandle
	(referred from /extra-gpios:num-gpios[0])

Both are due to it assuming that the 'num-gpios' property holds a phandle
pointing to a GPIO node.

To avoid these warnings, rename the sandbox property so that it does not
include the string 'gpios'.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/arch/sandbox/dts/test.dts b/arch/sandbox/dts/test.dts
index c38e369..b0f0ca8 100644
--- a/arch/sandbox/dts/test.dts
+++ b/arch/sandbox/dts/test.dts
@@ -177,7 +177,7 @@
 		gpio-controller;
 		#gpio-cells = <1>;
 		gpio-bank-name = "a";
-		num-gpios = <20>;
+		sandbox,gpio-count = <20>;
 	};
 
 	gpio_b: extra-gpios {
@@ -185,7 +185,7 @@
 		gpio-controller;
 		#gpio-cells = <5>;
 		gpio-bank-name = "b";
-		num-gpios = <10>;
+		sandbox,gpio-count = <10>;
 	};
 
 	i2c@0 {