ath79: add eth0 mac and initvals for Engenius EPG5000

ath79: add eth0 mac and initvals for Engenius EPG5000

Although VLANs are used, the "eth0" device by itself
does not have a valid MAC, so fix that with preinit script.

More initvals added by editing the driver to print switch registers,
after the bootloader sets them but before openwrt changes them.

The register bits needed for the QCA8337 switch
can be read from interrupted boot (tftpboot, bootm)
by adding print lines in the switch driver ar8327.c
before 'qca,ar8327-initvals' is parsed from DTS and written
for example:

  pr_info("0x04 %08x\n", ar8xxx_read(priv, AR8327_REG_PAD0_MODE));

Signed-off-by: Michael Pratt <mcpratt@pm.me>