ARM: imx6: DHCOM i.MX6 PDK: enable pad pull ups of SS lines of spi used for boot

It turned out that after a reset the boot process from the spi bootflash is
disturbed by other spi slave devices connected to DHCOM SPI1, which uses the
same spi interface with a different SS line.

Therefore the pad pull ups are enabled.

Signed-off-by: Ludwig Zenz <lzenz@dh-electronics.com>