clk: at91: add slow clks driver

AT91 slow clk is a clk multiplexer.

In some SoCs (sam9x5, sama5, sam9g45 families) this multiplexer can
choose among 2 sources: an internal RC oscillator circuit and an oscillator
using an external crystal.

In other Socs (sam9260 family) the multiplexer source is hardcoded with
the OSCSEL signal.

Signed-off-by: Boris BREZILLON <boris.brezillon@free-electrons.com>
Acked-by: Mike Turquette <mturquette@linaro.org>
Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>

drivers/clk/at91/Makefile

@@ -2,8 +2,8 @@
 # Makefile for at91 specific clk
 #
 
-obj-y += pmc.o
-obj-y += clk-main.o clk-pll.o clk-plldiv.o clk-master.o
+obj-y += pmc.o sckc.o
+obj-y += clk-slow.o clk-main.o clk-pll.o clk-plldiv.o clk-master.o
 obj-y += clk-system.o clk-peripheral.o clk-programmable.o
 
 obj-$(CONFIG_HAVE_AT91_UTMI)		+= clk-utmi.o