MIPS: ath79: Fix the ar913x reference clock rate

The reference clock on ar913x is at 40MHz and not 5MHz. The current
implementation use the wrong reference rate because it doesn't take
the PLL divider in account. But if we fix the code to use the divider
it becomes identical with the implementation for ar724x, so just drop
the broken ar913x implementation.

Signed-off-by: Alban Bedel <albeu@free.fr>
Tested-by: Antony Pavlov <antonynpavlov@gmail.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/12871/


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