net: Add option to disable fiber on M88E1111 PHY for PPC4xx

By defining CONFIG_M88E1111_DISABLE_FIBER boards can configure the
M88E1111 PYH to disable fiber. This is needed for an upcoming PPC460GT
based board, which has fiber/copper auto-selection enabled by default.
This doesn't seem to work. So we disable fiber in the PHY register.

Signed-off-by: Stefan Roese <sr@denx.de>
Signed-off-by: Ben Warren <biggerbadderben@gmail.com>