net/mlx5e: Prepare for using multiple TX doorbells

The driver allocates a single doorbell per device and uses
it for all Send Queues (SQs). This can become a bottleneck due to the
high number of concurrent MMIO accesses when ringing the same doorbell
from many channels.

This patch makes the doorbells used by channel queues configurable.

mlx5e_channel_pick_doorbell() is added to select the doorbell to be used
for a given channel, picking the default for now.

When opening a channel, the selected doorbell is saved to the channel
struct and used whenever channel-related queues are created.

Finally, 'uar_page' is added to 'struct mlx5e_create_sq_param' to
control which doorbell to use when allocating an SQ, since that can
happen outside channel context (e.g. for PTP).

Signed-off-by: Cosmin Ratiu <cratiu@nvidia.com>
Reviewed-by: Dragos Tatulea <dtatulea@nvidia.com>
Signed-off-by: Tariq Toukan <tariqt@nvidia.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>