omap3_beagle: Update for DM SPL support

- Switch to using the omap3-u-boot.dtsi file for needed properties.
- Remove a few SPL features to free up more SRAM space.
- Switch CONFIG_SYS_TEXT_BASE to the normal default, we don't need to
  worry about X-Loader at this point anymore.
- A few related updates to SPL options as part of switching to DM SPL.

Signed-off-by: Tom Rini <trini@konsulko.com>
Tested-by: Derald D. Woods <woods.technical@gmail.com>