clk: mvebu: Expand mv98dx3236-core-clock support

The initial implementation in commit e120c17a ("clk: mvebu: support
for 98DX3236 SoC") hardcoded a fixed value for the main PLL frequency.
Port code from the Marvell supplied Linux kernel to support different
PLL frequencies and provide clock gating support.

Signed-off-by: Chris Packham <chris.packham@alliedtelesis.co.nz>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>