configs: imx: Remove CONFIG_SYS_SPD_BUS_NUM option

According to the README:

"- CONFIG_SYS_SPD_BUS_NUM
		If SPD EEPROM is on an I2C bus other than the first
		one, specify here. Note that the value must resolve
		to something your driver can deal with."

There is no SPD EEPROM on the imx boards, so ged rid of this option.

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Acked-by: Stefano Babic <sbabic@denx.de>