board/ti/am335x: beaglebone stop muxing i2c1_pin_mux

On the BeagleBone these i2c1 pins are routed to the expanasion header, where
they can be defined as either pr1_usart0_Xxd/pwm0/spi0/i2c1, dont assume i2c1

Fixes: https://e2e.ti.com/support/arm/sitara_arm/f/791/p/313894/1387696



Signed-off-by: Robert Nelson <robertcnelson@gmail.com>
Reported-by: Matthijs van Duin <matthijsvanduin@gmail.com>
CC: Tom Rini <trini@konsulko.com>