tune-octeonx2: Do not use -mcpu=octeontx2 with clang

tune-octeonx2: Do not use -mcpu=octeontx2 with clang

-mcpu=octeontx2 is not implemented in clang, therefore
replace it with -march option denoting underlying
armv8 architecture revision.

 Fixes:

    |   error: unsupported argument 'octeontx2+crypto' to option '-mcpu='
    |   error: unknown target CPU 'octeontx2'

Signed-off-by: Khem Raj <raj.khem@gmail.com>
Signed-off-by: Mathieu Dubois-Briand <mathieu.dubois-briand@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>