am335x: add initial AM335x IDK board support

This patch extends the am335x_evm board for the AM335x IDK.

The IDK board uses MII for the ethernet phy (same as
Beaglebone board) and MMC0 for storage (but without
card detect line).

The IDK uses UART3 for console. So u-boot must be build
with CONFIG_SERIAL4 and CONFIG_CONS_INDEX=4 or for
the am335x_evm_uart3 board configuration as introduced
by Andrew Bradfords recent patch series "am33xx: Enable
UART {1,2,3,4,5}...". When using the IDK with console on UART0,
those patches are not required. In this case the board
slightly needs to be modified.

Signed-off-by: Matthias Fuchs <matthias.fuchs@esd.eu>