Up until now this driver only worked with data cache disabled.
To make it work with enabled data cache following changes were required:

 * Flush Tx/Rx buffer descriptors their modification
 * Invalidate Tx/Rx buffer descriptors before reading its values
 * Flush cache for data passed from CPU to GMAC
 * Invalidate cache for data passed from GMAC to CPU

Cc: Joe Hershberger <joe.hershberger@ni.com>
Cc: Vipin Kumar <vipin.kumar@st.com>
Cc: Stefan Roese <sr@denx.de>
Cc: Mischa Jonker <mjonker@synopsys.com>
Cc: Shiraz Hashim <shiraz.hashim@st.com>
Cc: Albert ARIBAUD <albert.u.boot@aribaud.net>
Cc: Amit Virdi <amit.virdi@st.com>
Cc: Sonic Zhang <sonic.zhang@analog.com>
Signed-off-by: Alexey Brodkin <abrodkin@synopsys.com>