ARM: am3517-evm: Migrate to SPL_OF_CONTROL

Like the other Logic PD OMAP35/DM37 boards, this board has device
tree enabled for U-Boot.  This patch converts the board to enable
SPL_OF_CONTROL and further shrinks the device tree in SPL to limit
it to UART3 (console), MMC1, i2c1, and GPIO4 (for mmc1 CD and WP).

There appears to be a bug in minicom so users may need to
switch the minicom terminal emulation to ANSI from VT102 due
to the junk that gets pushed out of the UART on startup.

Signed-off-by: Adam Ford <aford173@gmail.com>