am335x, guardian: adapt guardian board to DM

  - update partition table - remove env partitions
  - dts: add new interfaces (uart2, extra gpio-key)
         remove unneeded entries
         update nand timings for performance improvement
  - defconfig: adapt configurations to suit DM
               remove unneeded configs
  - am335x_guardian.h: remove mmc boot

Signed-off-by: Moses Christopher <BollavarapuMoses.Christopher@in.bosch.com>
Reviewed-by: Tom Rini <trini@konsulko.com>