Add a driver to support the special LDO access used by spring. This is a
custom method in the cros_ec protocol - it does not use an I2C
pass-through.
There are two implementation choices:
1. Write a special LDO driver which can talk across the EC. Duplicate all
the logic from TPS65090 for retrying when the LDO fails to come up.
2. Write a special I2C bus driver which pretends to be a TPS65090 and
transfers reads and writes using the LDO message.
Either is distasteful. The latter method is chosen since it results in less
code duplication and a fairly simple (30-line) implementation of the core
logic.
The crosec 'ldo' subcommand could be removed (since i2c md/mw will work
instead) but is retained as a convenience.
Signed-off-by: Simon Glass <sjg@chromium.org>
