PCI: exynos: Fix a potential init_clk_resources NULL pointer dereference

In order to avoid triggering a NULL pointer dereference in
exynos_pcie_probe() a check must be put in place to detect if
the init_clk_resources hook is initialized before calling it.

Add the respective function pointer check in exynos_pcie_probe().

Signed-off-by: Jaehoon Chung <jh80.chung@samsung.com>
[lorenzo.pieralisi@arm.com: rewrote the commit log]
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>