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

The Globalscale Technologies Sheevaplug board has the network chip
Marvell 88E1116R. Use uclass mvgbe and the compatible driver M88E1310
driver to bring up Ethernet.

- Remove CONFIG_RESET_PHY_R symbol from all board files
- Use uclass mvgbe to bring up the network. And remove ad-hoc code.
- Enable CONFIG_PHY_MARVELL to properly configure the network.
- Miscellaneous changes: Move constants to .c file and remove header file
board/Marvell/sheevaplug/sheevaplug.h, use BIT macro, and add/cleanup
comments.

Signed-off-by: Tony Dinh <mibodhi@gmail.com>