ARM: dts: rza2mevb: Add 48MHz USB clock

The RZ/A2M EVB has a 48MHz clock attached to USB_X1.

Signed-off-by: Chris Brandt <chris.brandt@renesas.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>