unhandled else in arch_timer_reg_read_cp15

Clang complains about return value being undefined which could happen if enum
value was corrupted.

Author: Mark Charlebois <charlebm@gmail.com>
Signed-off-by: Mark Charlebois <charlebm@gmail.com>

arch/arm64/include/asm/arch_timer.h

@@ -80,6 +80,8 @@ u32 arch_timer_reg_read_cp15(int access, enum arch_timer_reg reg)
 			asm volatile("mrs %0, cntv_tval_el0" : "=r" (val));
 			break;
 		}
+	} else {
+		BUG();
 	}
 
 	return (u32)val;