MAINTAINERS: add initial version

MAINTAINERS contains all currently known custodians based on
infos from wiki [1] and u-boot git forks [2].

[1] http://www.denx.de/wiki/U-Boot/Custodians
[2] http://git.denx.de/?p=u-boot.git;a=forks



Signed-off-by: Daniel Schwierzeck <daniel.schwierzeck@gmail.com>