CHROMIUM: mfd: chromeos_ec: instantiate sub-devices from device tree

CHROMIUM: mfd: chromeos_ec: instantiate sub-devices from device tree

If the EC device tree node has sub-nodes, try to instantiate them as MFD
sub-devices.
We can configure the EC features provided by the board.

Signed-off-by: Vincent Palatin <vpalatin@chromium.org>
BUG=chrome-os-partner:17812
TEST=on Spring, check that flashrom is working to re-flash the EC
firmware.

Change-Id: Ib1501eb3408cc13acb05073928ee958cd94f9107
Reviewed-on: https://gerrit.chromium.org/gerrit/43783
Reviewed-by: Todd Broch <tbroch@chromium.org>
Tested-by: Vincent Palatin <vpalatin@chromium.org>
Commit-Queue: Vincent Palatin <vpalatin@chromium.org>