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>