ARM: shmobile: Initial r8a7791 SoC support

Add initial support for the r8a7791 SoC including:
 - Single Cortex-A15 CPU Core
 - GIC

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: Hisashi Nakamura <hisashi.nakamura.ak@renesas.com>
Signed-off-by: Ryo Kataoka <ryo.kataoka.wt@renesas.com>
[damm@opensource.se: Forward ported to upstream, dropped not-yet-ready code]
Signed-off-by: Magnus Damm <damm@opensource.se>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>