ravb: Separate handling of irq enable/disable regs into feature

Currently, when the HW has a single interrupt, the driver uses the
GIC, TIC, RIC0 registers to enable and disable interrupts.
When the HW has multiple interrupts, it uses the GIE, GID, TIE, TID,
RIE0, RID0 registers.

However, other devices, e.g. RZ/V2M, have multiple irqs and only have
the GIC, TIC, RIC0 registers.
Therefore, split this into a separate feature.

Signed-off-by: Phil Edworthy <phil.edworthy@renesas.com>
Reviewed-by: Biju Das <biju.das.jz@bp.renesas.com>
Reviewed-by: Sergey Shtylyov <s.shtylyov@omp.ru>
Signed-off-by: David S. Miller <davem@davemloft.net>