mx6sxsabreauto: Remove legacy CONFIG_PCA953X

When this board was switched to using more DM drivers we didn't disable
the legacy PCA953X driver.  This in turn learn to a build time warning
about implicit functions as i2c.h would not say anything about
'i2c_read' nor 'i2c_write'.  But this was not a fatal error as none of
the legacy driver would be linked in either.

Fixes: e389033f ("imx: mx6sxsabreauto: enable more dm drivers")
Reviewed-by: Stefano Babic <sbabic@denx.de>
Signed-off-by: Tom Rini <trini@konsulko.com>