pfla02: Rework excluding NAND from SPL

Rather than only enable CONFIG_CMD_NAND for non-SPL builds, move the CMD
options to defconfig and rework the guards to not try and call the
function in SPL builds.

Cc: Stefano Babic <sbabic@denx.de>
Signed-off-by: Tom Rini <trini@konsulko.com>
---
A grep around for setup_gpmi_nand() shows that other platforms will just
make an empty function in similar situations.