samples: pwm: Add overlay for nucleo_l4r5zi board

Add Devicetree overlay for sample drivers/led/pwm and fix the one in
basic/blinky_pwm for nucleo_l4r5zi board to set appropriate to enable
pwdleds and set appropriate timer prescaler.

Signed-off-by: Yishai Jaffe <yishai1999@gmail.com>