microblaze: Add PVR for Microblaze v8.20.a

Microblaze v8.20.a has 0x15 version string.

Signed-off-by: Michal Simek <[e-mail]>

arch/microblaze/kernel/cpu/cpuinfo.c

···
 	{"8.00.a", 0x12},
 	{"8.00.b", 0x13},
 	{"8.10.a", 0x14},
+	{"8.20.a", 0x15},
 	{NULL, 0},
 };