PCI: endpoint: Cast the page number to phys_addr_t

Modify pci_epc_mem_alloc_addr() to cast the variable 'pageno'
from type 'int' to 'phys_addr_t' before shifting left. This
cast is needed to avoid treating bit 31 of 'pageno' as the
sign bit which would otherwise get sign-extended to produce
a negative value. When added to the base address of PCI memory
space, the negative value would produce an invalid physical
address which falls before the start of the PCI memory space.

Signed-off-by: Alan Mikhak <alan.mikhak@sifive.com>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Acked-by: Kishon Vijay Abraham I <kishon@ti.com>