boards: amlogic: add Khadas VIM3L support

Khadas VIM3L uses the same board layout as VIM3, but with an S905D3 chip
instead of A311D. Board config is derived from khadas-vim3_defconfig and
sei610_defconfig. README is based on README.khadas-vim3; the difference
is that VIM3L uses FIP files from the g12a folder in vendor sources not
the g12b folder.

Signed-off-by: Christian Hewitt <christianshewitt@gmail.com>
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
[narmstrong: added vim3l readme into w400 MAINTAINERS]