am335x_evm: Update SPI_BOOT support, add MTDPARTS info

- Style cleanup (# define -> #define)
- Due to ROM issues, redudant loading isn't feasible, so drop.
- Given extra space, increase max size of U-Boot to 512KiB
- Correct env size to match usage (we had not re-defined ENV_SIZE).
- Given extra space, keep env size as 128KiB, add redundant environment.

Reviewed-by: Peter Korsgaard <jacmet@sunsite.dk>
Signed-off-by: Tom Rini <trini@ti.com>