video: mb862xx: improve board-specific Lime configuration

To avoid board-specific code accessing the mb862xx registers directly,
the public function mb862xx_probe() has been introduced. Furthermore,
the "Change of Clock Frequency" and "Set Memory I/F Mode" registers
are now defined by CONFIG_SYS_MB862xx_CCF and CONFIG_SYS_MB862xx__MMR,
respectively. The BSPs for the socrates and lwmon5 boards have been
adapted accordingly.

Signed-off-by: Wolfgang Grandegger <wg@denx.de>