I2C master operating in multimaster mode can get stuck
indefinitely if I2C start is detected on bus, but no master
has a transaction going.

This is a weakness in I2C standard, which defines no way
to recover, since all masters are indefinitely disallowed
from interrupting the currently operating master. A start
condition can be created for example by an electromagnetic
discharge applied near physical I2C lines. Or a already
operating master could get reset immediately after sending
a start.

If it is known during device tree creation that only a single
I2C master will be present on the bus, this deadlock of the
I2C bus could be avoided in the driver by ignoring the
bus_is_busy register of the xiic, since bus can never be
reserved by any other master.

This patch adds this support for detecting single-master flag
in device tree and when provided, improves I2C reliability by
ignoring the therefore unnecessary xiic bus_is_busy register.

Error can be reproduced by pulling I2C SDA -line temporarily low
by shorting it to ground, while linux I2C master is operating on
it using the xiic driver. The application using the bus will
start receiving linux error code 16: "Device or resource busy"
indefinitely:

kernel: pca953x 0-0020: failed writing register
app: Error writing file, error: 16

With multi-master disabled device will instead receive error
code 5: "I/O error" while SDA is grounded, but recover normal
operation once short is removed.

kernel: pca953x 0-0020: failed reading register
app: Error reading file, error: 5
Signed-off-by: Jaakko Laine <ext-jaakko.laine@vaisala.com>
Acked-by: Michal Simek <michal.simek@xilinx.com>
Signed-off-by: Wolfram Sang <wsa@kernel.org>