arm: mx6: tqma6: Update to optionally configure an alternative SPI setup

By making the tqma6_iomuxc_spi() weak, this patch adds the possibility to
add a different function for this SPI configuration. This can be used
by other baseboards, that might have a different SPI setup.

This patch will be used by the upcoming WRU-IV board support which also
uses the TQMa6 SoM.

Signed-off-by: Stefan Roese <sr@denx.de>