mlxsw: spectrum_router: Introduce low-level ops and implement them for RALXX regs

In preparation for support of XM router implementation which uses
different registers to work with trees and FIB entries, introduce
a structure to hold low-level ops and implement tree manipulation
register ops.

Signed-off-by: Jiri Pirko <jiri@nvidia.com>
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>