ARM: ux500: Remove mop500_snowball_ethernet_clock_enable()

mop500_snowball_ethernet_clock_enable() provided a means to enable a
clock which was used for the SMSC911x Ethernet device on Snowball. It
was merely a stand-in until the driver was common clk compliant. Now
that it is, this can be removed for both DT and ATAGs booting.

Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Linus Walleij <linus.walleij@linaro.org>