clk: mark clk_disable_unused() as __init

clk_disable_unused is only called once, as a late_initcall, so reclaim
a bit of memory by marking it (and the functions and data it is the
sole user of) as __init/__initdata. This moves ~1900 bytes from .text
to .init.text for a imx_v6_v7_defconfig.

Signed-off-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Link: https://lkml.kernel.org/r/20191004094826.8320-1-linux@rasmusvillemoes.dk
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>