powerpc/powernv/idle: Restore CIABR after idle for Power9

On Power9, CIABR is lost after idle. This means that instruction
breakpoints set by xmon which use CIABR do not work. Fix this by
restoring CIABR after idle.

Signed-off-by: Jordan Niethe <jniethe5@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20201207010519.15597-2-jniethe5@gmail.com