ARM: shmobile: Initial r8a7790 SoC support

Add initial support for the r8a7790 SoC including:
 - Single Cortex-A15 CPU Core
 - GIC
 - Architecture timer

No static virtual mappings are used, all the components
make use of ioremap(). DT_MACHINE_START is still wrapped
in CONFIG_USE_OF to match other mach-shmobile code.

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