The Atheros AR7241 has 20 GPIO pins

AR724X_GPIO_PINS used for this family is defined as 18
The datasheet for the AR7241 describes 20 pins, allow all to be used.

Submitted by: Hiroki Mori <yamori813@yahoo.co.jp>
Reviewed by: mizhka
Differential Revision: https://reviews.freebsd.org/D17580