Hook up HSCIF serial devices that are part of the CPG/MSTP Clock Domain
to the SYSC "always-on" PM Domain, for a more consistent
device-power-area description in DT.

Cfr. commit 751e29bb ("ARM: dts: r8a7779: Use SYSC "always-on"
PM Domain").

Fixes: 055d15a8 ("ARM: dts: r8a7779: Add HSCIF0/1 device nodes")
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Simon Horman <horms+renesas@verge.net.au>
Reviewed-by: Ulrich Hecht <uli+renesas@fpond.eu>