[MIPS] Add basic SMARTMIPS ASE support

This patch adds trivial support for SMARTMIPS extension. This extension
is currently implemented by 4KS[CD] CPUs.

Basically it saves/restores ACX register, which is part of the SMARTMIPS
ASE, when needed. This patch does *not* add any support for Smartmips MMU
features.

Futhermore this patch does not add explicit support for 4KS[CD] CPUs since
they are respectively mips32 and mips32r2 compliant.  So with the current
processor configuration, a platform that has such CPUs needs to select
both configs:

	CPU_HAS_SMARTMIPS
	SYS_HAS_CPU_MIPS32_R[12]

This is due to the processor configuration which is mixing up all the
architecture variants and the processor types.

The drawback of this, is that we currently pass '-march=mips32' option to
gcc when building a kernel instead of '-march=4ksc' for 4KSC case. This
can lead to a kernel image a little bit bigger than required.

Signed-off-by: Franck Bui-Huu <fbuihuu@gmail.com>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

arch/mips/kernel/traps.c

@@ -229,6 +229,9 @@ void show_regs(struct pt_regs *regs)
 			printk("\n");
 	}
 
+#ifdef CONFIG_CPU_HAS_SMARTMIPS
+	printk("Acx    : %0*lx\n", field, regs->acx);
+#endif
 	printk("Hi    : %0*lx\n", field, regs->hi);
 	printk("Lo    : %0*lx\n", field, regs->lo);