arm64: dts: r8a7796 dtsi: Add all HSCIF nodes

Add the device nodes for all HSCIF serial ports, incl. clocks, and
power domain.

Signed-off-by: Ulrich Hecht <ulrich.hecht+renesas@gmail.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
[simon: express register size in hex; refer to power domain in changelog]
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>