MIPS: Kconfig: Set default MIPS system type as generic

MIPS: Kconfig: Set default MIPS system type as generic

The generic MIPS system type allows building a board agnostic kernel and
should be the default starting point for users, so set it as the default
system type in Kconfig.

Since ip22 is no longer the default, update ip22_defconfig to select
CONFIG_SGI_IP22.

Signed-off-by: Matt Redfearn <matt.redfearn@imgtec.com>
Cc: Ralf Baechle <ralf@linux-mips.org>
Cc: David Howells <dhowells@redhat.com>
Cc: Borislav Petkov <bp@suse.de>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/14714/
Signed-off-by: James Hogan <jhogan@kernel.org>