ARM: dts: am335x-evmsk: Add vtt_fixed regulator

The VTT regulator for DDR3 termination on the am335x-evmsk is
controlled by a gpio. It is configured by the bootloader so here we
define an always-on, fixed voltage regulator to hold the gpio.

Signed-off-by: Dave Gerlach <d-gerlach@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>