MIPS: ralink: fix spis group pinmux

MIPS: ralink: fix spis group pinmux

pwm function for spis conflicts with uart2 and uart1, fix this by changing it
to pwm_uart2, which reflects the real use of these pins with these pinmux
(2 for pwm and 2 for uart).

Signed-off-by: Álvaro Fernández Rojas <noltari@gmail.com>
Cc: john@phrozen.org
Cc: linux-mips@linux-mips.org
Cc: linux-kernel@vger.kernel.org
Patchwork: https://patchwork.linux-mips.org/patch/13369/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>