ODROID-U2: Adding regulators specific hack for U2
exynos: i2c: Leave the bus disabled unless it is in use
There is a rather odd feature of the exynos i2c controller that if it
is left enabled, it can lock itself up with the clk line held low.
This makes the bus unusable.

Unfortunately, the s3c24xx_i2c_set_master() function does not notice
this, and reports a timeout. From then on the bus cannot be used until
the AP is rebooted.

The problem happens when any sort of interrupt occurs (e.g. due to a
bus transition) when we are not in the middle of a transaction. We
have seen many instances of this when U-Boot leaves the bus apparently
happy, but Linux cannot access it.

The current code is therefore pretty fragile.

This fixes things by leaving the bus disabled unless we are actually
in a transaction. We enable the bus at the start of the transaction and
disable it at the end. That way we won't get interrupts and will not
lock up the bus.

It might be possible to clear pending interrupts on start-up, but this
seems to be a more robust solution. We can't service interrupts when
we are not in a transaction, and anyway would rather not lock up the
bus while we try.

BUG=chrome-os-partner:11124
TEST=manual:
Look up bus 4 in U-Boot by manually modifying the code to leave a
pending interrupt in the controller (stopping a transaction half-way
through with an error).

Boot into Linux and see that the bus is recovered and works normally
with:

$ i2cdump  -f -y 4 0x48

Without this change, we see:

s3c-i2c s3c2440-i2c.4: cannot get bus (error -110)

in the kernel logs.

Check that the keyboard works also (which uses i2c4).

Change-Id: Iccc9777fa8576bd4491dfbfbad044cec3638debb
Signed-off-by: Simon Glass <sjg@chromium.org>