powerpc: Initialize btext subsystem later, after prom_init

We were initializing the btext stuff from prom_init(), thus breaking
the rule that all communication between prom_init() and the rest of
the kernel has to be via the flattened device tree.  This removes
the btext initialization calls from prom_init() and initializes it
instead after the device tree is unflattened.  It would be nice to
do it earlier, but that needs some more infrastructure to find the
properties we need in the flattened device tree.

Signed-off-by: Paul Mackerras <paulus@samba.org>