powerpc/pasemi: Only the build the pasemi MSI code for PASEMI=y

The pasemi MSI code is currently always built when MPIC=y && PCI_MSI=y.
It should not have any effect on other platforms, because it immediately
checks the MPIC's compatible property for "pasemi,pwrficient-openpic".

However it's odd that it's still built even when PASEMI=n. It also
needn't be in sysdev, as it's only used by pasemi. So move it into
platforms/pasemi, whereby it will only be built for PASEMI=y.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>