net: stmmac: add support for independent DMA pbl for tx/rx

GMAC and newer supports independent programmable burst lengths for
DMA tx/rx. Add new optional devicetree properties representing this.

To be backwards compatible, snps,pbl will still be valid, but
snps,txpbl/snps,rxpbl will override the value in snps,pbl if set.

If the IP is synthesized to use the AXI interface, there is a register
and a matching DT property inside the optional stmmac-axi-config DT node
for controlling burst lengths, named snps,blen.
However, using this register, it is not possible to control tx and rx
independently. Also, this register is not available if the IP was
synthesized with, e.g., the AHB interface.

Signed-off-by: Niklas Cassel <niklas.cassel@axis.com>
Acked-by: Alexandre Torgue <alexandre.torgue@st.com>
Signed-off-by: David S. Miller <davem@davemloft.net>