tests: drivers: pwm: Support RA2, RA4, RA6 to test pwm_loopback

Add overlay to support test pwm_loopback on boards: ek_ra2a1,
ek_ra4e2, ek_ra4m2, ek_ra4m3, ek_ra4w1, ek_ra6e2, ek_ra6m1,
ek_ra6m2, ek_ra6m3, ek_ra6m4, ek_ra6m5, fpb_ra6e1, fpb_ra6e2.

Signed-off-by: Danh Doan <danh.doan.ue@bp.renesas.com>