amd-xgbe: add support for giant packet size

amd-xgbe: add support for giant packet size

AMD XGBE hardware supports giant Ethernet frames up to 16K bytes.
Add support for configuring and enabling giant packet handling
in the driver.

- Define new register fields and macros for giant packet support.
- Update the jumbo frame configuration logic to enable giant
  packet mode when MTU exceeds the jumbo threshold.

Acked-by: Shyam Sundar S K <Shyam-sundar.S-k@amd.com>
Signed-off-by: Raju Rangoju <Raju.Rangoju@amd.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/20250701121929.319690-1-Raju.Rangoju@amd.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>