microblaze: Add missing v8.50.a version

Add PVR value for MB 8.50.a.
Signed-off-by: Michal Simek <michal.simek@xilinx.com>

arch/microblaze/kernel/cpu/cpuinfo.c

@@ -40,6 +40,7 @@ const struct cpu_ver_key cpu_ver_lookup[] = {
 	{"8.30.a", 0x17},
 	{"8.40.a", 0x18},
 	{"8.40.b", 0x19},
+	{"8.50.a", 0x1a},
 	{"9.0", 0x1b},
 	{"9.1", 0x1d},
 	{NULL, 0},