Fillup ravb_alloc_rx_desc_gbeth() function to support RZ/G2L.

This patch also renames ravb_alloc_rx_desc to ravb_alloc_rx_desc_rcar
to be consistent with the naming convention used in sh_eth driver.

Signed-off-by: Biju Das <biju.das.jz@bp.renesas.com>
Reviewed-by: Lad Prabhakar <prabhakar.mahadev-lad.rj@bp.renesas.com>
Reviewed-by: Sergey Shtylyov <s.shtylyov@omp.ru>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>