CLK: microchip: Add Microchip PIC32 clock driver.

CLK: microchip: Add Microchip PIC32 clock driver.

This clock driver implements PIC32 specific clock-tree. clock-tree
entities can only be configured through device-tree file (OF).

Signed-off-by: Purna Chandra Mandal <purna.mandal@microchip.com>
Cc: Michael Turquette <mturquette@baylibre.com>
Cc: Stephen Boyd <sboyd@codeaurora.org>
Cc: linux-kernel@vger.kernel.org
Cc: linux-mips@linux-mips.org
Cc: linux-clk@vger.kernel.org
Patchwork: https://patchwork.linux-mips.org/patch/13247/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>