boards: renesas: add on-board button support on Renesas RA boards

Add gpio keys support for these boards: ek_ra2a1, ek_ra4e2, ek_ra4m2,
ek_ra4m3, ek_ra4w1, ek_ra6e2, ek_ra6m1, ek_ra6m2, ek_ra6m3, ek_ra6m4,
ek_ra6m5, ek_ra8d1, ek_ra8m1, fpb_ra6e1, fpb_ra6e2

Signed-off-by: The Nguyen <the.nguyen.yf@renesas.com>