ARM: dts: stm32: fix dmamux reg property on stm32mp151

Reg property length should cover all DMAMUX_CxCR registers.
DMAMUX_CxCR Address offset: 0x000 + 0x04 * x (x = 0 to 15), so latest
offset is at 0x3c, so length should be 0x40.
Signed-off-by: Amelie Delaunay <amelie.delaunay@st.com>
Signed-off-by: Alexandre Torgue <alexandre.torgue@st.com>

arch/arm/boot/dts/stm32mp151.dtsi

@@ -1001,7 +1001,7 @@ dma2: dma-controller@48001000 {
 
 		dmamux1: dma-router@48002000 {
 			compatible = "st,stm32h7-dmamux";
-			reg = <0x48002000 0x1c>;
+			reg = <0x48002000 0x40>;
 			#dma-cells = <3>;
 			dma-requests = <128>;
 			dma-masters = <&dma1 &dma2>;