[POWERPC] Use udbg_early_init() on ppc32

udbg_early_init() is a function used on 64 bit systems, which
initializes whichever early udbg backend is configured.  This function
is not called on 32-bit, however if btext early debug is enabled it
does have an explicit, inline, #ifdef-ed assignment performing
analagous initialization.

This patch makes things more uniform by folding the btext
initialization as an option into udbg_early_init() and calling that
from the 32-bit setup path.

Signed-off-by: David Gibson <dwg@au1.ibm.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>