ramips: drop obsolete sd card driver code

ramips: drop obsolete sd card driver code

The pinmux for all SoCs using this driver is now set via the pinmux. It
makes this code obsolete.

Some of the code targeting the mt76x8 SoCs is still required. The sd
card pins share the pads with the EPHY. These pads need to be switched
to digital mode if the pins are used for sd cards.

The eMMC 8-bit mode has to be enabled via pinmux instead of a kernel
option. The uart2 group need to be set to function "sdxc d5 d4", pwm1
to "sdxc d6" and pwm0 to "sdxc d7" to do so. It can't be done by as
part of a default pinmux, as it would break the normal operation of
uart2.

Signed-off-by: Mathias Kresin <dev@kresin.me>
(backported from 287b7aa583584262a68fcebc3786779b2ab14307)