boards: amlogic: add Odroid C4 support

Odroid C4 is an Amlogic SM1 device, the board config and board documentation
are adapted from the Odroid-N2 support from the same vendor.

Signed-off-by: Christian Hewitt <christianshewitt@gmail.com>
[narmstrong: fix odroid-c4.rst typos and structure]
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Tested-by: Anand Moon <linux.amoon@gmail.com>