MIPS: Preserve Config implementation-defined bits

The coprocessor 0 Config register includes 9 implementation defined
bits, which in some processors do things like enable write combining or
other functionality. We ought not to wipe them to 0 during boot. Rather
than doing so, preserve their value & only clear the bits standardised
by the MIPS architecture.

Signed-off-by: Paul Burton <paul.burton@imgtec.com>

arch/mips/cpu/start.S

@@ -123,8 +123,9 @@ reset:
 	mtc0	zero, CP0_COMPARE
 
 #ifndef CONFIG_SKIP_LOWLEVEL_INIT
-	/* CONFIG0 register */
-	li	t0, CONF_CM_UNCACHED
+	mfc0	t0, CP0_CONFIG
+	and	t0, t0, MIPS_CONF_IMPL
+	or	t0, t0, CONF_CM_UNCACHED
 	mtc0	t0, CP0_CONFIG
 #endif
 

arch/mips/include/asm/mipsregs.h

@@ -450,6 +450,7 @@
 #define MIPS_CONF_MT_FTLB	(_ULCAST_(4) <<  7)
 #define MIPS_CONF_AR		(_ULCAST_(7) << 10)
 #define MIPS_CONF_AT		(_ULCAST_(3) << 13)
+#define MIPS_CONF_IMPL		(_ULCAST_(0x1ff) << 16)
 #define MIPS_CONF_M		(_ULCAST_(1) << 31)
 
 /*