sparc/PCI: Use default pcibios_enable_device() (Leon only)

We don't need anything arch-specific in pcibios_enable_device() so drop
the arch implementation and use the default generic one.

Note that sparc has two pcibios_enable_device() implementations other than
the one removed here.

Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
CC: Daniel Hellstrom <daniel@gaisler.com