powerpc: fix spelling mistake: "Usupported" -> "Unsupported"

Trivial fix to spelling mistake in bootx_printf message text
Signed-off-by: NColin Ian King <colin.king@canonical.com>
Signed-off-by: NMichael Ellerman <mpe@ellerman.id.au>

arch/powerpc/platforms/powermac/bootx_init.c

@@ -519,7 +519,7 @@ void __init bootx_init(unsigned long r3, unsigned long r4)
 			;
 	}
 	if (bi->architecture != BOOT_ARCH_PCI) {
-		bootx_printf(" !!! WARNING - Usupported machine"
+		bootx_printf(" !!! WARNING - Unsupported machine"
 			     " architecture !\n");
 		for (;;)
 			;