net: xilinx: axi_emac: Fix dma descriptors for 64bit and compilation warnings

There are compilation warnings showing up when we compile AXI ethernet
driver for 64bit architectures. Fix them, so that it works on both 32
and 64 bit architectures.

DMA descriptors are not taking care of 64bit addresses. To fix it,
change axidma_bd members as below:

next		==>	next_desc
reserverd1	==>	next_desc_msb
phys		==>	buf_addr
reserverd2	==>	buf_addr_msb

and update next_desc and buf_addr with lower 32 bits of the addresses,
update next_desc_msb and buf_addr_msb with upper 32 bits of the 64bit
addresses.

Signed-off-by: Ashok Reddy Soma <ashok.reddy.soma@xilinx.com>
Reviewed-by: Ramon Fried <rfried.dev@gmail.com>