powerpc/ofw: Fix malloc inside lock

Summary:
Currently ofw_real_bounce_alloc() is requesting memory, using WAITOK, holding a
non-sleepable locks, called 'OF Bounce Page'.

Fix this by allocating the pages outside of the lock, and only updating the
global variables while holding the lock.

Submitted by: Breno Leitao
Differential Revision: https://reviews.freebsd.org/D14955