net: ethernet: litex: add support for 64 bit stats

Implement 64 bit per cpu stats to fix the overflow of netdev->stats
on 32 bit platforms. To simplify the code, we use net core
pcpu_sw_netstats infrastructure. One small drawback is some memory
overhead because litex uses just one queue, but we allocate the
counters per cpu.

Signed-off-by: Jisheng Zhang <jszhang@kernel.org>
Reviewed-by: Simon Horman <simon.horman@corigine.com>
Acked-by: Gabriel Somlo <gsomlo@gmail.com>
Link: https://lore.kernel.org/r/20230614162035.300-1-jszhang@kernel.org


Signed-off-by: Jakub Kicinski <kuba@kernel.org>