PCI: imx6: Simplify imx7d_pcie_wait_for_phy_pll_lock()

Make use of regmap_read_poll_timeout() to simplify
imx7d_pcie_wait_for_phy_pll_lock(). No functional change intended.

Signed-off-by: Andrey Smirnov <andrew.smirnov@gmail.com>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Reviewed-by: Lucas Stach <l.stach@pengutronix.de>
Cc: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Cc: Bjorn Helgaas <bhelgaas@google.com>
Cc: Chris Healy <cphealy@gmail.com>
Cc: Lucas Stach <l.stach@pengutronix.de>
Cc: linux-kernel@vger.kernel.org
Cc: linux-pci@vger.kernel.org