i2c: ocores: add polling interface

This driver assumes that an interrupt line is always available for
the I2C master. This is not always the case and this patch adds support
for a polling version.

Report from Andrew Lunn:

  I did some timing tests for this. On my box, we request a udelay of
  80uS. The kernel actually delays for about 79uS. We then spin in
  ocores_wait() for an additional 10-11uS, which is 3 to 4 iterations.

  There are actually 9 bits on the wire, not 8, since there is an
  ACK/NACK bit after the actual data transfer. So i changed the delay to
  (9 * 1000) / i2c->bus_clock_khz. That resulted in ocores_wait() mostly
  not looping at all. But for reading an 4K AT24 EEPROM, it increased
  the read time by 10ms, from 424ms to 434ms. So we should probably keep
  with 8.

Signed-off-by: Federico Vaga <federico.vaga@cern.ch>
Tested-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>