of: refactor of_modalias_node() and remove explicit match table.

This patch tightens up the behaviour of of_modalias_node() to be more
predicatable and to eliminate the explicit of_modalias_tablep[] that
is currently used to override the first entry in the compatible list
of a device.  The override table was needed originally because spi
and i2c drivers had no way to do of-style matching.  Now that all
devices can have an of_node pointer, and all drivers can have an
of_match_table, the explicit override table is no longer needed
because each driver can specify its own OF-style match data.

The mpc8349emitx-mcu driver is modified to explicitly specify the
correct device to bind against.

Signed-off-by: Grant Likely <grant.likely@secretlab.ca>