There's light at the end of the tunnel 
An inquisitive soul found out the following kernel symbol went AWOL in the migration from kernel 4.14 to 5.4:
diff --git a/target/linux/ramips/mt7621/config-5.4 b/target/linux/ramips/mt7621/config-5.4
index fba22a39cf..559abb65f5 100644
--- a/target/linux/ramips/mt7621/config-5.4
+++ b/target/linux/ramips/mt7621/config-5.4
@@ -210,6 +205,7 @@ CONFIG_PHYLINK=y
CONFIG_PINCTRL=y
CONFIG_PINCTRL_RT2880=y
# CONFIG_PINCTRL_SINGLE is not set
+CONFIG_PINCTRL_SX150X=y
CONFIG_POWER_RESET=y
CONFIG_POWER_RESET_GPIO=y
CONFIG_POWER_SUPPLY=y
Props to @svanheule for tracking this down.
If you compile yourself, you can just add this patch, if not, you'll have to be patient. It turns out this driver cannot be compiiled as a module so there's some input from the devs needed on how to proceed, since this would enable it for all MT7621 devices.