cmd: mvebu: Rename rx_training to mvebu_comphy_rx_training

Rename the misleading cmd "rx_training" to "mvebu_comphy_rx_training" to
avoid confusion and mixup with DDR3/4 training. This makes it clear,
that this command is platform specific and handles the COMPHY RX
training.

Also depend this cmd on ARMADA_8K and not TARGET_MVEBU_ARMADA_8K to make
is available for OcteonTX2 CN913x.

Signed-off-by: Stefan Roese <sr@denx.de>
Cc: Pali Rohár <pali@kernel.org>
Cc: Marek Behun <marek.behun@nic.cz>
Cc: Kostya Porotchkin <kostap@marvell.com>
Cc: Nadav Haklai <nadavh@marvell.com>
Acked-by: Marek Behún <marek.behun@nic.cz>
Acked-by: Pali Rohár <pali@kernel.org>