spi: kirkwood_spi: Add support for multiple chip-selects on MVEBU

spi: kirkwood_spi: Add support for multiple chip-selects on MVEBU

Currently only chip-select 0 is supported by the kirkwood SPI driver.
The Armada XP / 38x SoCs also use this driver and support multiple chip
selects. This patch adds support for multiple CS on MVEBU.

The register definitions are restructured a bit with this patch. Grouping
them to the corresponding registers.

Signed-off-by: Stefan Roese <sr@denx.de>
Cc: Luka Perkov <luka.perkov@sartura.hr>
Reviewed-by: Jagan Teki <jteki@openedev.com>