ARM: dts: samsung: smdk4412: align keypad node names with dtschema

DT schema expects certain node names, as pointed out but dtbs_check:

  exynos4412-smdk4412.dtb: keypad@100a0000: 'key-A', 'key-B', 'key-C', 'key-D', 'key-E' do not match any of the regexes: '^key-[0-9a-z]+$', 'pinctrl-[0-9]+'

Link: https://lore.kernel.org/r/20240312183105.715735-4-krzysztof.kozlowski@linaro.orgSigned-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>

arch/arm/boot/dts/samsung/exynos4412-smdk4412.dts

@@ -105,31 +105,31 @@ key-5 {
 		linux,code = <6>;
 	};
 
-	key-A {
+	key-a {
 		keypad,row = <2>;
 		keypad,column = <6>;
 		linux,code = <30>;
 	};
 
-	key-B {
+	key-b {
 		keypad,row = <2>;
 		keypad,column = <7>;
 		linux,code = <48>;
 	};
 
-	key-C {
+	key-c {
 		keypad,row = <0>;
 		keypad,column = <5>;
 		linux,code = <46>;
 	};
 
-	key-D {
+	key-d {
 		keypad,row = <2>;
 		keypad,column = <5>;
 		linux,code = <32>;
 	};
 
-	key-E {
+	key-e {
 		keypad,row = <0>;
 		keypad,column = <7>;
 		linux,code = <18>;