ARM: mx23: dynamically allocate mx23 auart device

i.MX23 Reference Manaul starts auart index from 1 than 0.  Changing
the index to start from 0 requires corresponding changes on base
address, irq, and iomux definitions, and will probably confuse people
who reads codes and hardware documents together.

This patch introduced the field 'hwid' to distinguish the driver id
and controller id.  These two ids are different on mx23 while
identical on mx28.

Signed-off-by: Shawn Guo <shawn.guo@freescale.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>