net: zynq: Add a note about RX_BUF macro

Record note about reducing number of BDs.
Signed-off-by: NMichal Simek <michal.simek@xilinx.com>

drivers/net/zynq_gem.c

@@ -173,6 +173,7 @@ struct emac_bd {
 #endif
 };
 
+/* Reduce amount of BUFs if you have limited amount of memory */
 #define RX_BUF 32
 /* Page table entries are set to 1MB, or multiples of 1MB
  * (not < 1MB). driver uses less bd's so use 1MB bdspace.