ARM: mx6: Disable PCIe on SABRE Lite/Nitrogen6x

Use of PCIe on SABRE Lite and Nitrogen6x boards
is atypical and requires the use of custom daughter
boards.

Use in U-Boot is even rarer, so this patch removes it from
the standard configuration.

Signed-off-by: Eric Nelson <eric.nelson@boundarydevices.com>
Acked-by: Marek Vasut <marex@denx.de>