am335x-boneblue.dts: fix eQEP0 pinmux

Fixing a simple typo for one of the eQEP0 pins and is supported by the code comment on the relevant line and bone Blue schematics.

Testing on the board itself has only gone so far as getting no conflict with an overlay that muxes the (incorrect) pin for a different purpose. Before the fix a conflict would occur. v4.19 based builds did not exhibit this conflict.