MIPS: ath79: select the PINCTRL subsystem

The pinmux on QCA SoCs is controlled by a single register. The
"pinctrl-single" driver can be used but requires the target
to select PINCTRL.

Signed-off-by: John Crispin <john@phrozen.org>
Signed-off-by: Paul Burton <paul.burton@mips.com>
Patchwork: https://patchwork.linux-mips.org/patch/19909/
Cc: James Hogan <jhogan@kernel.org>
Cc: Ralf Baechle <ralf@linux-mips.org>
Cc: linux-mips@linux-mips.org