net: ti: icssg-prueth: Add ethtool ops for Frame Preemption MAC Merge

Add driver support for viewing / changing the MAC Merge sublayer
parameters and dump the Mac Merge stats via ethtool ops: .set_mm(),
.get_mm() and .get_mm_stats().

The minimum size of non-final mPacket fragments supported by the firmware
without leading errors is 64 Bytes (in octets). Add a check to ensure
user passed tx_min_frag_size argument via ethtool, honors this .
Add pa stats registers to check statistics for preemption, which can be
dumped using ethtool ops.

Signed-off-by: Meghana Malladi <m-malladi@ti.com>
Signed-off-by: MD Danish Anwar <danishanwar@ti.com>