PCI: spear: Add __init annotation to spear13xx_add_pcie_port()

Add __init annotation to spear13xx_add_pcie_port(), because
spear13xx_add_pcie_port() is called only by spear13xx_pcie_probe(), which
is marked __init.  This fixes a section mismatch warning:

WARNING: drivers/pci/host/built-in.o(.text.unlikely+0x94): Section mismatch in reference from the function spear13xx_add_pcie_port() to the function .init.text:dw_pcie_host_init()

Signed-off-by: Jingoo Han <jg1.han@samsung.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>