PCI: endpoint: Replace spinlock with mutex

The pci_epc_ops is not intended to be invoked from interrupt context.
Hence replace spin_lock_irqsave and spin_unlock_irqrestore with
mutex_lock and mutex_unlock respectively.

Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>