uboot-phytec

kw_spi: add weak functions board_spi_claim/release_bus

This allows a final, board specific, step in the claim/relase_bus
function for the SPI controller, which may be needed for some hardware
designs.

Signed-off-by: Valentin Longchamp <valentin.longchamp@keymile.com>
cc: Holger Brunck <holger.brunck@keymile.com>
cc: Prafulla Wadaskar <prafulla@marvell.com>