kirkwood: remove get_random_hex() and MD5 dependency

kirkwood: remove get_random_hex() and MD5 dependency

Commit 19a5944fcd62 ("mvgbe: remove setting of ethaddr within the
driver") removed the usage of get_random_hex() from the mvgbe driver
about six years ago. However the prototype of that function survived
till today in some kirkwood header file.
Remove that prototype and the CONFIG_MD5 dependency triggered by that.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Stefan Roese <sr@denx.de>