clk: mvebu Add the time base generator clocks for Armada 3700

These clocks are children of the xtal clock and each one can be selected
as a source for the peripheral clocks.

According to the datasheet it should be possible to modify their rate,
but currently it is not supported.

Signed-off-by: Gregory CLEMENT <gregory.clement@free-electrons.com>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>