clk: mvebu: Add clk support for the orion5x SoC mv88f5181

Referring to the u-boot sources for the Netgear WNR854T, add support
for the mv88f5181.

[gregory.clement@free-electrons.com: fix commit title]
Signed-off-by: Jamie Lentin <jm@lentin.co.uk>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Acked-by: Rob Herring <robh@kernel.org>
Acked-by: Stephen Boyd <sboyd@codeaurora.org>
Signed-off-by: Gregory CLEMENT <gregory.clement@free-electrons.com>