ARM: dts: am3517-evm-uboot: Add reg-shift for UART

With the resync of the omap3.dtsi file, the reg-shift was removed
so it breaks the UART.  Adding the reg-shift into the
am3517-evm-u-boot.dtsi keeps the reg-shift for U-Boot, but keeps
the dts/dtsi files clean from Linux.

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