tegra: trimslice: set up serial flash pinmux

When Trimslice is booted from serial flash, the boot ROM does this, so
U-Boot doesn't need to. However, booting from the SD slot for recovery
purposes, the boot ROM does not set up the pinmux for serial flash. Add
code to U-Boot to set this up, so that an SD-based recovery U-Boot image
can upgrade the U-Boot in serial flash.

Signed-off-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Tom Warren <twarren@nvidia.com>