keymile: Move sourcing of common Kconfig

The way board/keymile/Kconfig is written protects the options there from
being parsed on non-keymile platforms.  We cannot however safely source
this file from multiple locations.  This does not manifest as a problem
currently as there are no choice statements inside of this file (nor the
sub-Kconfig files it sources).  However, moving some target selection to
one of these files exposes the underlying problem.  Rework things so
that we have this file sourced in arch/Kconfig.

Cc: Holger Brunck <holger.brunck@hitachienergy.com>
Signed-off-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Holger Brunck <holger.brunck@hitachienergy.com>
diff --git a/arch/Kconfig b/arch/Kconfig
index 8450fdb..bc31e5a 100644
--- a/arch/Kconfig
+++ b/arch/Kconfig
@@ -448,3 +448,5 @@
 source "arch/x86/Kconfig"
 source "arch/xtensa/Kconfig"
 source "arch/riscv/Kconfig"
+
+source "board/keymile/Kconfig"