ramips: mt7621: enable SX150x driver

ramips: mt7621: enable SX150x driver

The Netgear R6800 and R6700v2 devices have a Semtech SX1503 GPIO
expander controlling the device LEDs. This expander was initially
supported on 4.14, but support was lost in the transition to 5.4.

Since this driver cannot be built as a kernel module, enable it in the
kernel config for all mt7621 devices.

Run-tested on a Netgear R6800.

Cc: Stijn Segers <foss@volatilesystems.org>
Cc: Hauke Mehrtens <hauke@hauke-m.de>
Signed-off-by: Sander Vanheule <sander@svanheule.net>
Tested-by: Stijn Segers <foss@volatilesystems.org>