riscv: sifive: initialize PCI on Unmatched

The Unmatched board is typically booted from NVMe which requires PCI.
When dropping to a console PCI is not initialized yet. 'pci enum' has to be
called.

Change the configuration to call  pci_init() in board_init_r().

Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Reviewed-by: Rick Chen <rick@andestech.com>