clk: fix comments in include/clk.h

The comment about get_periph_rate() is the same as that of
set_periph_rate().

I am fixing typos here and there while I am in this file.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Acked-by: Simon Glass <sjg@chromium.org>