net: ethernet: ti: Fix mixed module-builtin object

net: ethernet: ti: Fix mixed module-builtin object

With CONFIG_TI_K3_AM65_CPSW_NUSS=y and CONFIG_TI_ICSSG_PRUETH=m,
k3-cppi-desc-pool.o is linked to a module and also to vmlinux even though
the expected CFLAGS are different between builtins and modules.

The build system is complaining about the following:

k3-cppi-desc-pool.o is added to multiple modules: icssg-prueth
ti-am65-cpsw-nuss

Introduce the new module, k3-cppi-desc-pool, to provide the common
functions to ti-am65-cpsw-nuss and icssg-prueth.

Fixes: 128d5874c082 ("net: ti: icssg-prueth: Add ICSSG ethernet driver")
Signed-off-by: MD Danish Anwar <danishanwar@ti.com>
Link: https://lore.kernel.org/r/20231018064936.3146846-1-danishanwar@ti.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>