clk: at91: move slow clock controller clocks to sckc.c

Move all clocks related to the slow clock controller to sckc.c. This avoids
extern definitions and allows to remove sckc.h

Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
[sboyd@codeaurora.org: Mark some functions static]
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>