ARM: socfpga: dts: Add div-reg to the main_pll clocks

The mpu_clk, main_clk, and dbg_base_clk outputs from the main PLL go through a
pre-divider. Update socfpga.dtsi to represent those dividers for these
clocks.

Re-use the "div-reg" property that was used for the socfpga-gate-clock as this
is the same thing. Also update the documentation.

Signed-off-by: Dinh Nguyen <[e-mail]>

Documentation/devicetree/bindings/clock/altr_socfpga.txt

···
 - fixed-divider : If clocks have a fixed divider value, use this property.
 - clk-gate : For "socfpga-gate-clk", clk-gate contains the gating register
         and the bit index.
-- div-reg : For "socfpga-gate-clk", div-reg contains the divider register, bit shift,
-        and width.
+- div-reg : For "socfpga-gate-clk" and "socfpga-periph-clock", div-reg contains
+	the divider register, bit shift, and width.
 - clk-phase : For the sdmmc_clk, contains the value of the clock phase that controls
 	the SDMMC CIU clock. The first value is the clk_sample(smpsel), and the second
 	value is the cclk_in_drv(drvsel). The clk-phase is used to enable the correct

arch/arm/boot/dts/socfpga.dtsi

···
 							#clock-cells = <0>;
 							compatible = "altr,socfpga-perip-clk";
 							clocks = <&main_pll>;
-							fixed-divider = <2>;
+							div-reg = <0xe0 0 9>;
 							reg = <0x48>;
 						};
 
···
 							#clock-cells = <0>;
 							compatible = "altr,socfpga-perip-clk";
 							clocks = <&main_pll>;
-							fixed-divider = <4>;
+							div-reg = <0xe4 0 9>;
 							reg = <0x4C>;
 						};
 
···
 							#clock-cells = <0>;
 							compatible = "altr,socfpga-perip-clk";
 							clocks = <&main_pll>;
-							fixed-divider = <4>;
+							div-reg = <0xe8 0 9>;
 							reg = <0x50>;
 						};