arm64/hwcap: Document allocation of upper bits of AT_HWCAP

The top two bits of AT_HWCAP are reserved for use by glibc and the rest of
the top 32 bits are being kept unallocated for potential use by glibc.
Document this in the header.

Signed-off-by: Mark Brown <broonie@kernel.org>
Acked-by: Catalin Marinas <catalin.marinas@arm.com>
Acked-by: Szabolcs Nagy <szabolcs.nagy@arm.com>
Link: https://lore.kernel.org/r/20220707103632.12745-2-broonie@kernel.org
Signed-off-by: Will Deacon <will@kernel.org>

arch/arm64/include/uapi/asm/hwcap.h

@@ -19,6 +19,9 @@
 
 /*
  * HWCAP flags - for AT_HWCAP
+ *
+ * Bits 62 and 63 are reserved for use by libc.
+ * Bits 32-61 are unallocated for potential use by libc.
  */
 #define HWCAP_FP		(1 << 0)
 #define HWCAP_ASIMD		(1 << 1)