mv643xx_eth: configurable t_clk

Make t_clk configurable via platform device data (with the current
hardcoded value, 133 MHz, being the default), as it varies across
different chip families.

Signed-off-by: Lennert Buytenhek <buytenh@marvell.com>
Acked-by: Nicolas Pitre <nico@marvell.com>
Signed-off-by: Dale Farnsworth <dale@farnsworth.org>