MIPS: pic32mzda: fix getting timer clock rate.

PIC32 clock driver is now implemented as platform driver instead of
as part of of_clk_init(). It meants all the clock modules are available
quite late in the boot sequence. So request for CPU clock by clk_get_sys()
and clk_get_rate() to find c0_timer rate fails.

To fix this use PIC32 specific early clock functions implemented for early
console support.

Signed-off-by: Purna Chandra Mandal <purna.mandal@microchip.com>
Cc: linux-kernel@vger.kernel.org
Cc: linux-mips@linux-mips.org
Cc: Joshua Henderson <digitalpeer@digitalpeer.com>
Patchwork: https://patchwork.linux-mips.org/patch/13262/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>