Documentation: dt: atmel-at91: add slow clock to tcb

The timer counters need the slow clock. It is required as they
will not function without it.

Signed-off-by: Boris Brezillon <[e-mail]>
Signed-off-by: Alexandre Belloni <[e-mail]>

Documentation/devicetree/bindings/arm/atmel-at91.txt

···
   Note that you can specify several interrupt cells if the TC
   block has one interrupt per channel.
 - clock-names: tuple listing input clock names.
-	Required elements: "t0_clk"
+	Required elements: "t0_clk", "slow_clk"
 	Optional elements: "t1_clk", "t2_clk"
 - clocks: phandles to input clocks.