mips: ralink: make SOC_MT7621 select PINCTRL_MT7621 and fix help section

The dt-binding of the MT7621 SoC uses the pinctrl driver. Therefore, have
this driver selected by default.

The help section for SOC_MT7621 exceeds 80 columns which is the preferred
limit of a single line. A tab character is 8 columns. Rearrange the text
accordingly.

Signed-off-by: Arınç ÜNAL <arinc.unal@arinc9.com>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>