m68k: enabled software emulation of separate supervisor/user stack

Recent Coldfires have separate supervisor and user stack pointers, but
since older Coldfires didn't have that, the Linux kernel has a kind of
emulation mechanism for those pointers.

Apparently, according to the Kconfig.cpu file, the 5208 is supposed to
support such separate pointers, but Qemu doesn't implement it. So we
cheat a bit here and force the usage of emulated separate stack
pointers.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni at free-electrons.com>

diff -Nur linux-3.16.orig/arch/m68k/Kconfig.cpu linux-3.16/arch/m68k/Kconfig.cpu
--- linux-3.16.orig/arch/m68k/Kconfig.cpu	2014-08-04 00:25:02.000000000 +0200
+++ linux-3.16/arch/m68k/Kconfig.cpu	2014-08-13 16:58:59.568332805 +0200
@@ -146,6 +146,7 @@
 	depends on !MMU
 	select GENERIC_CLOCKEVENTS
 	select HAVE_CACHE_SPLIT
+	select COLDFIRE_SW_A7
 	help
 	   Freescale Coldfire 5207/5208 processor support.