port the am335x based shc board to DM, to get rid
of DW warnings when compiling U-Boot.

- remove uneccessary board code
- adapt defconfigs
- remove unneeded defconfigs
  configs/am335x_shc_prompt_defconfig
  configs/am335x_shc_sdboot_prompt_defconfig
Signed-off-by: NHeiko Schocher <hs@denx.de>
Reviewed-by: NTom Rini <trini@konsulko.com>