DW SPI: Allow to overload the management of the external CS

DW SPI: Allow to overload the management of the external CS

On some platforms, as the Ocelot ones, when wanting to control the CS
through software, it is not possible to do it through the GPIO
controller. Indeed, this signal is managed through a dedicated range of
registers inside the SoC..

By declaring the external_cs_manage function as weak, it is possible to
manage the CS at platform level and then using the appropriate registers.

Signed-off-by: Gregory CLEMENT <gregory.clement@bootlin.com>