[POWERPC] Add non-OF serial console support

Add serial console support for non-OF systems.  There is a generic serial
console layer which calls a serial console driver.  Included is the serial
console driver for the ns16550 class of uarts.  Necessary support routines
are added as well.

Signed-off-by: Mark A. Greer <mgreer@mvista.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>