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If AMU is not supported by the hardware but it is enabled in Trusted
Firmware, the console will be spammed with warnings every time a CPU
is brought up with a CPU ON call.

Remove the warning message as this is more in line with how other
extensions like SPE and SVE are handled.

Change-Id: Iba6d367e4d1375ab554d23d2eaceab3ae1362c5a
Signed-off-by: Dimitris Papastamos <dimitris.papastamos@arm.com>