sandbox: Fix DT compiler pin warnings in sandbox DTs

sandbox: Fix DT compiler pin warnings in sandbox DTs

Trivially fix the following warnings in sandbox DTs, which show up with
DTC 1.7.2. Assign pin groups less confusing node names with pins- prefix
to avoid confusing DT compiler into thinking the node is really a bus node:

"
arch/sandbox/dts/.test.dtb.pre.tmp:1831.20-1841.5: Warning (i2c_bus_bridge): /pinctrl/i2c: incorrect #address-cells for I2C bus
arch/sandbox/dts/.test.dtb.pre.tmp:1831.20-1841.5: Warning (i2c_bus_bridge): /pinctrl/i2c: incorrect #size-cells for I2C bus
arch/sandbox/dts/test.dtb: Warning (i2c_bus_reg): Failed prerequisite 'i2c_bus_bridge'
arch/sandbox/dts/.test.dtb.pre.tmp:1848.20-1856.5: Warning (spi_bus_bridge): /pinctrl/spi: incorrect #address-cells for SPI bus
arch/sandbox/dts/.test.dtb.pre.tmp:1848.20-1856.5: Warning (spi_bus_bridge): /pinctrl/spi: incorrect #size-cells for SPI bus
arch/sandbox/dts/test.dtb: Warning (spi_bus_reg): Failed prerequisite 'spi_bus_bridge'
"

Signed-off-by: Marek Vasut <marek.vasut+renesas@mailbox.org>
Reviewed-by: Mattijs Korpershoek <mkorpershoek@kernel.org>
Reviewed-by: Heiko Schocher <hs@nabladev.com>