ARM: shmobile: Break out R-Car Gen2 setup code

Move arch timer workaround code and boot mode pin
handling from setup-r8a7790.c to setup-rcar-gen2.c.

With this in place the same code can be used on
other R-Car Generation 2 devices such as r8a7791.

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