Import i.MX6SL clock infrastructure from linux clk-imx6sl.c

To save space, clocks beeing unlikely usefull for bootloader purposes
(SSI, SPDIF, EXTERN_AUDIO) were not imported.
Further, the fixup code from linux mainline commits
a49e6c4b8204 ("ARM: imx: add common clock support for fixup mux")
cbe7fc8aaeef ("ARM: imx: add common clock support for fixup div")
was ignored for this commit.

Signed-off-by: Alexander Kurz <akurz@blala.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>