powerpc: PCI_MSI needs PCI

Various powerpc boards select the PCI_MSI config option without selecting
PCI, resulting in potentially not compilable configurations if the by
default enabled PCI option is disabled.  Explicitly select PCI to ensure
we always have valid configs.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Acked-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>