post/i2c: Add ability to ignore I2C devices

Add the ability to not report an I2C POST error for a set of given I2C
addresses on bootup.  This is useful for cases when a device may or may
not be present, and neither case is considered an error.  For example:

- Some form factors such as XMC and Compact PCI Express have an I2C
  EEPROM whose address changes based on geographical address.  Eg
  installed in one slot its EEPROM address is, 0x50, in another its
  0x51, etc.  This allows multiple devices to have their EEPROMs present
  on the same I2C bus.  Thus the I2C devices present for an XMC or
  CPCIe card depend on if and where other cards are installed in the
  same system.

- Some cards have optional I2C devices.  Eg one hardware build
  configuration has different I2C devices than another and software
  can't determine if the optional device should be present or not.

- Some cards have optional daughtercards with I2C devices on them.

- I2C EEPROMs address range depends on their size.  Its possible to
  support differently size EEPROMs by only probing the EEPROM's base
  address and ignoring the other addresses that are impacted by its
  size.

A new CONFIG_SYS_POST_I2C_IGNORES define has been added which specifies
a list of I2C addresses for the I2C POST to ignore.

Signed-off-by: Peter Tyser <ptyser@xes-inc.com>
Acked-by: Heiko Schocher <hs@denx.de>
Acked-by: Wolfgang Denk <wd@denx.de>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>