spl: initialize PCI before booting

MMC, SATA, and USB may be using PCI based controllers.
Initialize the PCI sub-system before trying to boot.

Remove the initialization for NVMe that is now redundant.

Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Reviewed-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Mayuresh Chitale <mchitale@ventanamicro.com>