PCI: designware-ep: Pre-allocate memory for MSI in dw_pcie_ep_init

Certain SoCs need to map the MSI address in raise_irq.
To map an address, you first need to call pci_epc_mem_alloc_addr(),
however, pci_epc_mem_alloc_addr() calls ioremap() (which can sleep).

Since raise_irq is only called from atomic context, we can't call
pci_epc_mem_alloc_addr() from raise_irq.

Pre-allocate a page in dw_pcie_ep_init(), so that this page can later
be used to map/unmap the MSI address in raise_irq.

Tested-by: Gustavo Pimentel <gustavo.pimentel@synopsys.com>
Signed-off-by: Niklas Cassel <niklas.cassel@axis.com>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Acked-by: Joao Pinto <jpinto@synopsys.com>