boards: renesas: Add power management support for RA8 boards

Updated DTS files to enable ULPT nodes for:
- `ek_ra8d1`
- `ek_ra8m1`
- `mck_ra8t1`

Updated board documentation in `index.rst` for each board.

Signed-off-by: Khanh Nguyen <[email protected]>

Author: KhanhNguyen-RVC

boards/renesas/ek_ra8d1/ek_ra8d1.dts

@@ -326,3 +326,19 @@ pmod_sd_shield: &sdhc1 {};
 &wdt {
 	status = "okay";
 };
+
+&ulpt0 {
+	status = "okay";
+
+	timer {
+		status = "okay";
+	};
+};
+
+&ulpt1 {
+	status = "okay";
+
+	timer {
+		status = "okay";
+	};
+};

boards/renesas/ek_ra8m1/ek_ra8m1.dts

@@ -385,3 +385,19 @@ pmod_sd_shield: &sdhc0 {};
 &wdt {
 	status = "okay";
 };
+
+&ulpt0 {
+	status = "okay";
+
+	timer {
+		status = "okay";
+	};
+};
+
+&ulpt1 {
+	status = "okay";
+
+	timer {
+		status = "okay";
+	};
+};

boards/renesas/mck_ra8t1/mck_ra8t1.dts

@@ -236,3 +236,19 @@
 &wdt {
 	status = "okay";
 };
+
+&ulpt0 {
+	status = "okay";
+
+	timer {
+		status = "okay";
+	};
+};
+
+&ulpt1 {
+	status = "okay";
+
+	timer {
+		status = "okay";
+	};
+};