am335x_evm: Convert CONFIG_CONS_INDEX into a menu choice

- Drop CONFIG_SERIAL[1-6] and use CONFIG_CONS_INDEX tests instead
- Add choice and help text to board/ti/am335x/Kconfig
- Correct comment about IDK in board/ti/am335x/mux.c
- Remove am335x_evm_uart* defconfig files as they're just variations
  on a config option now.

Signed-off-by: Tom Rini <trini@ti.com>