ti: add support for AM6254atl SiP

ti: add support for AM6254atl SiP

TI's AM6254atl (or AM62x SiP for short) provides the existing AM62x SoC
with 512MiB of DDR integrated in a single package.

This patch adds the necessary U-Boot devie tree files, the required
defconfigs along with the documentation for the AM62x SiP EVM.

AM62x SiP differs from the already supported AM62x in following ways:

- OP-TEE for the AM62x resides from 0x9e800000 to 0xa0000000 which needs
  to be moved to 0x80080000 to free up space at end of DDR in AM62x SiP
  with 512MiB of memory. This is required to allow U-Boot to relocate to
  end of DDR before booting to the kernel.

- Changes to the env:
   1. splashimage address updated from 0x80200000 to 0x81a00000
   2. DFU addresses updated to match updated TEXT_BASE for SPL and U-Boot

Signed-off-by: Anshul Dalal <anshuld@ti.com>