arm: kirkwood: GoFlex Home : Use Marvell uclass mvgbe and PHY driver for Ethernet

The GoFlex Home board has the network chip Marvell 88E1116R.
Use uclass mvgbe and the compatible driver M88E1118R to bring up Ethernet.

- Currently, CONFIG_RESET_PHY_R symbol is used in
arch/arm/mach-kirkwood/include/mach/config.h for all Kirkwood
boards with mv8831116 PHY, with each board defines the function
reset_phy(). Undefine it for this board.
- Add board_eth_init() to use uclass mvgbe to bring up the network.
And remove ad-hoc code.
- Enable CONFIG_PHY_MARVELL to properly configure the network.
- Miscellaneous changes: use CONFIG_SYS_THUMB_BUILD to keep u-boot image
under 512K, use BIT macro, and cleanup comments.

Signed-off-by: Tony Dinh <mibodhi@gmail.com>
Reviewed-by: Pali Rohár <pali@kernel.org>
Reviewed-by: Stefan Roese <sr@denx.de>