ppc: Adapt to asm-powerpc/irq.h irq_canonicalize changes

Now instead of having a ppc_md function, we just have a variable
which says whether to do the i8259 irq canonicalization or not,
and set that variable on the platforms that need that.  It looks
to me that radstone_ppc7d was trying to use irq canonicalization
for something else in a broken kind of way - it will need to be
fixed properly.

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