net: mvpp2: fix second cp110 initialization

Since the mdio code got upstreamed it is not possible
to activate network ports on CP110 Master and Slave.

The problem is in mvpp2_base_probe which is called for each
CP110 and it initializes the buffer area for descs and rx_buffers.

This should only happen once though and the bd space is actually
set to 0 after the first run of the function.

This leads to an error when the second CP110 tries the initialization
again and disables all network ports on this CP110.

This patch adds a static variable to check if the buffer area is
initialized only once globally.

Signed-off-by: Sven Auhagen <sven.auhagen@voleatech.de>
Reviewed-by: Stefan Roese <sr@denx.de>