mlxsw: reg: add rdpm register

Add rdpm definition - router DSCP to priority mapping register.
This register will be utilized later to align the default mapping between
packet DSCP and switch-priority to the kernel's mapping between
packet priority and skb priority.

This is the first non-bit indexed register where the entries are arranged
in descending order, i.e., entry at offset 0 matches configuration for
dscp[63]. As a result, the item's step is converted into a signed variable
to support descending arrays [where step would be negative].

Signed-off-by: Yuval Mintz <yuvalm@mellanox.com>
Signed-off-by: Nogah Frankel <nogahf@mellanox.com>
Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>