arm64: enable generic CPU feature modalias matching for this architecture

This enables support for the generic CPU feature modalias implementation that
wires up optional CPU features to udev based module autoprobing.

A file <asm/cpufeature.h> is provided that maps CPU feature numbers to
elf_hwcap bits, which is the standard way on arm64 to advertise optional CPU
features both internally and to user space.

Signed-off-by: Ard Biesheuvel <[e-mail]>
[[e-mail]: removed unnecessary "!!"]
Signed-off-by: Catalin Marinas <[e-mail]>

arch/arm64/Kconfig

···
 	select DCACHE_WORD_ACCESS
 	select GENERIC_CLOCKEVENTS
 	select GENERIC_CLOCKEVENTS_BROADCAST if SMP
+	select GENERIC_CPU_AUTOPROBE
 	select GENERIC_IOMAP
 	select GENERIC_IRQ_PROBE
 	select GENERIC_IRQ_SHOW

arch/arm64/include/asm/cpufeature.h

···
+/*
+ * Copyright (C) 2014 Linaro Ltd. <[e-mail]>
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License version 2 as
+ * published by the Free Software Foundation.
+ */
+
+#ifndef __ASM_CPUFEATURE_H
+#define __ASM_CPUFEATURE_H
+
+#include <asm/hwcap.h>
+
+/*
+ * In the arm64 world (as in the ARM world), elf_hwcap is used both internally
+ * in the kernel and for user space to keep track of which optional features
+ * are supported by the current system. So let's map feature 'x' to HWCAP_x.
+ * Note that HWCAP_x constants are bit fields so we need to take the log.
+ */
+
+#define MAX_CPU_FEATURES	(8 * sizeof(elf_hwcap))
+#define cpu_feature(x)		ilog2(HWCAP_ ## x)
+
+static inline bool cpu_have_feature(unsigned int num)
+{
+	return elf_hwcap & (1UL << num);
+}
+
+#endif