tests: drivers: pwm: Fix build

Commit a1f4821 ("soc: silabs: siwx91x: Rename pins") broke the test
for pwm_api.

Signed-off-by: Jérôme Pouiller <[email protected]>

Author: jerome-pouiller

tests/drivers/pwm/pwm_api/boards/siwx917_rb4338a.overlay

@@ -15,7 +15,7 @@
 &pinctrl0 {
 	pwm_ch: pwm_ch {
 		group1 {
-			pinmux = <PWM_0H_PA7>; /* GPIO_7 P20 */
+			pinmux = <PWM_0H_HP7>; /* GPIO_7 P20 */
 		};
 	};
 };