spi: mvebu_a3700_spi: Use Armada 37xx clk driver for SPI clock frequency

Since now we have driver for clocks on Armada 37xx, use it to determine
SQF clock frequency for the SPI driver.

Also change the default config files for Armada 37xx devices so that
the clock driver is enabled by default, otherwise the SPI driver cannot
be enabled.

Signed-off-by: Marek Behun <marek.behun@nic.cz>
Signed-off-by: Stefan Roese <sr@denx.de>
diff --git a/arch/arm/dts/armada-37xx.dtsi b/arch/arm/dts/armada-37xx.dtsi
index c72fd25..5b4a1a4 100644
--- a/arch/arm/dts/armada-37xx.dtsi
+++ b/arch/arm/dts/armada-37xx.dtsi
@@ -301,8 +301,8 @@
 				#address-cells = <1>;
 				#size-cells = <0>;
 				#clock-cells = <0>;
-				clock-frequency = <160000>;
-				spi-max-frequency = <40000>;
+				spi-max-frequency = <50000000>;
+				clocks = <&nb_periph_clk 7>;
 				status = "disabled";
 			};