net: dwc_eth_qos: Allow platform to override tx/rx_fifo_sz

The GMAC controller on QCS404 SoC (support added by upcoming patch) fails
to work with maximum tx/rx_fifo_sz supported by the hardware (16K). So
allow platforms to override FIFO size using corresponding DT node
properties.

Signed-off-by: Sumit Garg <sumit.garg@linaro.org>
Reviewed-by: Ramon Fried <rfried.dev@gmail.com>