ethernet: ucc_geth: remove bd_mem_part and all associated code

The bd_mem_part member of ucc_geth_info always has the value
MEM_PART_SYSTEM, and AFAICT, there has never been any code setting it
to any other value. Moreover, muram is a somewhat precious resource,
so there's no point using that when normal memory serves just as well.

Apart from removing a lot of dead code, this is also motivated by
wanting to clean up the "store result from kmalloc() in a u32" mess.

Signed-off-by: Rasmus Villemoes <rasmus.villemoes@prevas.dk>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>