net: ipa: greater timer granularity options

Starting with IPA v5.0, the head-of-line blocking timer has more
than two pulse generators available to define timer granularity.
To prepare for that, change the way the field value is encoded
to use ipa_reg_encode() rather than ipa_reg_bit().

The aggregation granularity selection could (in principle) also use
an additional pulse generator starting with IPA v5.0.  Encode the
AGGR_GRAN_SEL field differently to allow that as well.

Signed-off-by: Alex Elder <elder@linaro.org>
Reviewed-by: Leon Romanovsky <leonro@nvidia.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>