Add overlay and enable support for QCA7000 board

This adds a device tree overlay for the QCA7000 which can be used
when attaching an I2SE's PLC Stamp micro EVK to the Raspberry Pi.

This Evaluation Board embeds a QCA7000 chip, a Homeplug Green PHY
powerline chip from Qualcomm/Atheros for the Internet of Things.

This patch also enables the required QCA7000 driver module
in the default configurations.

Signed-off-by: Stefan Wahren <stefan.wahren@i2se.com>
Signed-off-by: Michael Heimpold <michael.heimpold@i2se.com>