I2C: mxc_i2c: make I2C1 and I2C2 optional

The driver assumed that I2C1 and I2C2 were always enabled,
and if they were not, then an asynchronous abort was (silently)
raised, to be caught much later on in the Linux kernel.

Fix this by making I2C1 and I2C2 optional just like I2C3 and I2C4
are.

To make the change binary-invariant, declare I2C1 and I2C2 in
every include/configs/ file which defines CONFIG_SYS_I2C_MXC.

Also, while updating README about CONFIG_SYS_I2C_MXC_I2C1 and
CONFIG_SYS_I2C_MXC_I2C2, add missing descriptions for I2C4 speed
(CONFIG_SYS_MXC_I2C4_SPEED) and slave (CONFIG_SYS_MXC_I2C4_SLAVE)
config options.

Signed-off-by: Albert ARIBAUD (3ADEV) <albert.aribaud@3adev.fr>