powerpc/512x: add function for chip select parameter configuration

Add ability to configure chip select (CS) parameters for devices
that need different CS parameters setup after their configuration.
I.e. an FPGA device on LP bus can require different CS parameters
for its bus interface after loading firmware into it. A driver
can easily reconfigure the LPC CS parameters using this function.

Acked-by: Timur Tabi <timur@tabi.org>
Signed-off-by: Anatolij Gustschin <agust@denx.de>