net: lpc-enet: move phy setup into platform code

Setting the phy mode requires touching a platform specific
register, which prevents us from building the driver without
its header files.

Move it into a separate function in arch/arm/mach/lpc32xx
to hide the core registers from the network driver.

Link: https://lore.kernel.org/r/20190809144043.476786-8-arnd@arndb.de


Acked-by: Sylvain Lemieux <slemieux.tyco@gmail.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>