net: ks8851: Add Kconfig entries

Convert CONFIG_KS8851_MLL and CONFIG_KS8851_MLL_BASEADDR to Kconfig

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Eugen Hristev <eugen.hristev@microchip.com>
Cc: Joe Hershberger <joe.hershberger@ni.com>