[POWERPC] Update g5_defconfig

Update the g5_defconfig with default settings.
This is to keep things up to date, and specifically to ensure that the
CONFIG_MACINTOSH_DRIVERS option is enabled.  This also turns on
CONFIG_MSI.

Signed-off-by: Will Schmidt <will_schmidt@vnet.ibm.com>
cc: Michael Ellerman <michael@ellerman.id.au>
Signed-off-by: Paul Mackerras <paulus@samba.org>