ARM: dts: r8a7791: Add GyroADC clock and device node

Add node for the GyroADC block and it's associated clock.

Signed-off-by: Marek Vasut <marek.vasut+renesas@gmail.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>