MIPS: AR7: Fix loops per jiffies on TNETD7200 devices

TNETD7200 run their CPU clock faster than the default CPU clock we assume.
In order to have the correct loops per jiffies settings, initialize clocks right
before setting mips_hpt_frequency. As a side effect, we can no longer use
msleep in clocks.c which requires other parts of the kernel to be initialized,
so replace these with mdelay.

Signed-off-by: Florian Fainelli <florian@openwrt.org>
To: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/1749/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>