powerpc/xmon: Enable breakpoints on 8xx

Since commit 4ad8622d ("powerpc/8xx: Implement hw_breakpoint"),
8xx has breakpoints so there is no reason to opt breakpoint logic
out of xmon for the 8xx.

Fixes: 4ad8622d ("powerpc/8xx: Implement hw_breakpoint")
Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/b0607f1113d1558e73476bb06db0ee16d31a6e5b.1608716197.git.christophe.leroy@csgroup.eu