ARM: dts: r8a7791: Rename the serial port clock to fck

The clock is really the device functional clock, not the interface
clock. Rename it.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>