Support for M4

- Remove UART3 snippets from the DTS. This is the UART for the M4, and
not used in u-boot anyways.
- Add TCM (tightly coupled memory) region in the board_lmb_reserve, otherwise we can't load
into the TCM region.
- Add definitions for TCM location and size
- Define commands for loading and running the M4 (called from boot
script)

Change-Id: I3a83fe7503418aeb2867aef10e9eeb5079165b94