Across all SoCs, even on Armada 370 for SPI0, the clock pin uses the
'sck' subname and not 'clk', so this commit adjusts the code and
documentation accordingly.

Since this commit only changes the subname, DT backward compatibility
is not affected.
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>