mips: ralink: make SOC_MT7621 select PINCTRL

Currently, out of every Ralink SoC, only the dt-binding of the MT7621 SoC
uses pinctrl. Because of this, PINCTRL is not selected at all. Make
SOC_MT7621 select PINCTRL.

Remove PINCTRL_MT7621, enabling it for the MT7621 SoC will be handled under
the PINCTRL_MT7621 option.

Signed-off-by: Arınç ÜNAL <arinc.unal@arinc9.com>
Acked-by: Sergio Paracuellos <sergio.paracuellos@gmail.com>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>