efi: Don't look for chosen@0 node on DT platforms

Due to some scary special case handling noticed in drivers/of, various
bits of the ARM* EFI support patches did duplicate looking for @0
variants of various nodes. Unless on an ancient PPC system, these are
not in fact required. Most instances have become refactored out along
the way, this removes the last one.

Signed-off-by: Leif Lindholm <leif.lindholm@linaro.org>
Signed-off-by: Matt Fleming <matt.fleming@intel.com>