boards: renesas: Add Flash HP support for Renesas RA6, RA4

- Add Flash HP support for ek_ra6m1, ek_ra6m2, ek_ra6m3, ek_ra6m4,
ek_ra6m5, ek_ra6e2, fpb_ra6e1, fpb_ra6e2, ek_ra4e2, ek_ra4m2,
ek_ra4m3
- Add doc about Flash for these boards

Signed-off-by: Khoa Nguyen <khoa.nguyen.xh@renesas.com>
Signed-off-by: Phi Tran <phi.tran.jg@bp.renesas.com>