ath79: reduce 'nvmem-cells' definitions on ALFA Network QCA9531 boards

All the QCA9531 based boards from ALFA Network are based on the same
design and share a common DTSI: 'qca9531_alfa-network_r36a.dtsi'.

Instead of defining 'nvmem-cells' for the MAC address in every device's
DTS, move definition to the common DTSI file.

Signed-off-by: Piotr Dymacz <pepe2k@gmail.com>