clk: davinci: New driver for davinci PLL clocks

This adds a new driver for mach-davinci PLL clocks. This is porting the
code from arch/arm/mach-davinci/clock.c to the common clock framework.
Additionally, it adds device tree support for these clocks.

The ifeq ($(CONFIG_COMMON_CLK), y) in the Makefile is needed to prevent
compile errors until the clock code in arch/arm/mach-davinci is removed.

Note: although there are similar clocks for TI Keystone we are not able
to share the code for a few reasons. The keystone clocks are device tree
only and use legacy one-node-per-clock bindings. Also the register
layouts are a bit different, which would add even more if/else mess
to the keystone clocks. And the keystone PLL driver doesn't support
setting clock rates.

Signed-off-by: David Lechner <david@lechnology.com>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>