keystone2: move cpu_to_bus() to keystone.c

The SoC related common functions in board.c should be placed to
a common keystone.c arch file.

Acked-by: Murali Karicheri <m-maricheri2@ti.com>
Signed-off-by: Hao Zhang <hzhang@ti.com>
Signed-off-by: Ivan Khoronzhuk <ivan.khoronzhuk@ti.com>