ARM: am3517_evm: Remove hard-coded pin muxing

With updated device trees now supporting pinmuxing for USB,
ethernet, MMC, and other peripherals necessary to start MLO
and U-Boot, the hard-coded pinmux options can be removed since
they are now handed by DM and only muxed when the respective
peripheral needs it.

Signed-off-by: Adam Ford <aford173@gmail.com>
Tested-by: Derald D. Woods <woods.technical@gmail.com>