clk: sunxi: gmac-tx-clk mux is not a CLK_MUX_INDEX_BIT mux

A CLK_MUX_INDEX_BIT mux has one bit per parent, but the sun7i-a20-gmac-clk
has 2 bits selecting between 3 possible parents using values of 0, 1, 2,
which makes it a regular mux which should not have CLK_MUX_INDEX_BIT set in
its flag.

However we do not support parent 1 (an external clock), so use a table to
select parent 0 or 2, which are the 2 parents we support.

Note this has not been causing any issues sofar, because we start with a
parent setting of parent 0, and only ever re-parent to parent 2 (for which
we use an index of 1 as we skip parent 1) and with CLK_MUX_INDEX_BIT set
we write a value of 2 for index 1.

Tested on both a cubietruck (which uses rgmii mode) as well as a cs908
(an a31s board which uses mii mode).

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>