ARM: shmobile: Add select MIGHT_HAVE_PCI for PCI-AHB bridge code

ARM: shmobile: Add select MIGHT_HAVE_PCI for PCI-AHB bridge code

The PCI sub-system is not enabled by default on ARM and on certain
Renesas devices the build does not select it. This means that there
are configurations that do not allow the AHB-PCI bridge used for the
USB sub-systems to be built.

For the R8A7790, R8A7791 and EMEV-2 select MIGHT_HAVE_PCI to allow the
PCI drivers to be built. Also select MIGHT_HAVE_PCI for the multi-config
where there may be many Reneasas devices selected.

Reviewed-by: Ian Molton <ian.molton@codethink.co.uk>
Signed-off-by: Ben Dooks <ben.dooks@codethink.co.uk>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>