ARM: shmobile: r8a7790 SCIF support

Add SCIF serial port support to the r8a7790 SoC by
adding platform devices for SCIFA0 -> SCIFA2 as well
as SCIFB0 -> SCIFB2 and SCIF0 -> SCIF1 together with
clock bindings. DT device description is excluded at
this point since such bindings are still under
development.

Signed-off-by: Magnus Damm <damm@opensource.se>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>