sh: fix i2c init order on Migo-R V2

Convert the Migo-R board code to register devices at
arch_initcall() time instead of __initcall(). This fix
unbreaks migor_ts touch screen driver support.

Signed-off-by: Magnus Damm <damm@igel.co.jp>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>