clk: add Dove PLL divider support for GPU, VMeta and AXI clocks

Add support for the Dove PLL dividers, which are used to generate the
clocks for the AXI bus, as well as the GPU and VMeta peripherals.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
Signed-off-by: Michael Turquette <mturquette@baylibre.com>