net, davinci_emac: make clock divider in MDIO control register configurable

Define CONFIG_SYS_EMAC_TI_CLKDIV for setting the clkdiv value
in the MDIO control register.

Signed-off-by: Heiko Schocher <hs@denx.de>
cc: Sandeep Paulraj <s-paulraj@ti.com>
cc: Wolfgang Denk <wd@denx.de>
Signed-off-by: Sandeep Paulraj <s-paulraj@ti.com>