sparc: Add mutex for set property calls.

On some platforms, the I2C controller is shared between the OS and
OBP.  OBP uses this I2C controller to access the EEPROM, and thus is
programmed when the kernel calls prom_setprop().

Wrap such calls with the new of_set_property_mutex.

Relevant I2C bus drivers can grab this mutex around top-level I2C
operations to provide the proper protection.

Signed-off-by: David S. Miller <davem@davemloft.net>