[POWERPC] Mark udbg console as CON_ANYTIME, ie. callable early in boot

The udbg console should be safe to call basically at any time after boot.
It does not need any per-cpu resources or for the cpu to be online, as
long as there is a udbg_putc routine hooked up it should work. So mark it
as CON_ANYTIME.

Signed-off-by: Michael Ellerman <michael@ellerman.id.au>
Signed-off-by: Paul Mackerras <paulus@samba.org>