MIPS: SEAD3: Remove custom read_persistent_clock

The SEAD3 board defines a custom implementation of read_persistent_clock
which does exactly the same dummy operation as the generic weak version.
Remove the not really implemented custom version.

Signed-off-by: Paul Burton <paul.burton@imgtec.com>
Cc: linux-mips@linux-mips.org
Cc: linux-kernel@vger.kernel.org
Patchwork: https://patchwork.linux-mips.org/patch/14064/


Signed-off-by: Ralf Baechle <ralf@linux-mips.org>