PCI: spear: Fix Section mismatch compilation warning for probe()

Following compilation warning occurs when compiled with:
CONFIG_DEBUG_SECTION_MISMATCH=y

 WARNING: drivers/pci/host/built-in.o(.data+0xc0): Section mismatch in
 reference from the variable spear13xx_pcie_driver to the function
 .init.text:spear13xx_pcie_probe()

Both .probe() and pcie_init() are marked with __init, but spear13xx_pcie_driver
isn't. And so section mismatch.

Fix it by marking spear13xx_pcie_driver with __initdata.

Fixes: 51b66a6c (PCI: spear: Add PCIe driver for ST Microelectronics SPEAr13xx)
Reported-by: Olof Johansson <olof@lixom.net>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Olof Johansson <olof@lixom.net>