ARM: keystone: Enable PINCTRL_SINGLE Config

Newer Keystone 2 devices utilize pinmuxing. Enable PINCTRL SINGLE
which is used to handle pinmuxing.

Signed-off-by: Franklin S Cooper Jr <fcooper@ti.com>
Signed-off-by: Keerthy <j-keerthy@ti.com>
Signed-off-by: Santosh Shilimkar <ssantosh@kernel.org>