net/designware: make driver compatible with data cache

net/designware: make driver compatible with data cache

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 <[email protected]>
Cc: Vipin Kumar <[email protected]>
Cc: Stefan Roese <[email protected]>
Cc: Mischa Jonker <[email protected]>
Cc: Shiraz Hashim <[email protected]>
Cc: Albert ARIBAUD <[email protected]>
Cc: Amit Virdi <[email protected]>
Cc: Sonic Zhang <[email protected]>
Signed-off-by: Alexey Brodkin <[email protected]>