boards: renesas: Add counter doc for RA6, RA4, RA2

Add doc for AGT counter for:
ek_ra6m1, ek_ra6m2, ek_ra6m3, ek_ra6m4, ek_ra6m5, fpb_ra6e1,
ek_ra4w1, ek_ra4m2, ek_ra4m3, ek_ra2a1.

Signed-off-by: Khoa Nguyen <khoa.nguyen.xh@renesas.com>