tune-cortexa72: Enable the crc extension by default for cortexa72

The crc extension is optional for the ARMv8.0 but is mandatory for the
cortexa72, so there is no reason not to enable it for the cortexa72
tune. With this change, the cortexa72-crc seems redundant. But we
had better to keep it to be compatible with the BSP which already used
that tune.

Signed-off-by: Kevin Hao <kexin.hao@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>