fsl_i2c: increase I2C timeout values and make them configurable

The value of I2C_TIMEOUT in fsl_i2c.c has several problems.  First, it is
defined as CONFIG_HZ/4, but it is used as a count of microseconds, so it makes
no sense to derive it from a clock rate.  Second, the current value (250) is
too low for some boards, so it needs to be increased.  Third, the timeout
necessary for multiple-master arbitration is larger than the timeout for basic
read/write operations, so we shouldn't have a single constant for both timeouts.
Finally, it would be nice if we could override these values on a per-board
basis.

Signed-off-by: Timur Tabi <timur@freescale.com>
Acked-by: Wolfgang Denk <wd@denx.de>
Tested-by: Peter Tyser <ptyser@xes-inc.com>
Acked-by: Peter Tyser <ptyser@xes-inc.com>