powerpc/mpc7448: add alias list to DTS, clean out old chosen node

The mpc7448hpc2 board doesn't have an alias block like
most of the other modern eval boards have.  We need this
block in order to have u-boot be able to make use of the
CONFIG_OF_STDOUT_VIA_ALIAS (vs. having a hard coded node)
in the future.

Also remove the old, redundant chosen node.  Of all the modern
Freescale eval boards (incl. 83xx, 85xx, 86xx) this is the only
one which still has it.  Its presence also breaks with some older
versions of u-boot, like 1.3.1 -- which try and insert a
second chosen node.

Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>