phy: marvell: a3700: Set TXDCLK_2X_SEL bit during PCIe initialization

Marvell Armada 3700 Functional Specifications, section 52.2 PCIe Link
Initialization says that TXDCLK_2X_SEL bit needs to be enabled for PCIe
Root Complex mode.

Same change was included in TF-A project:
https://review.trustedfirmware.org/c/TF-A/trusted-firmware-a/+/9408



Signed-off-by: Pali Rohár <pali@kernel.org>
Reviewed-by: Stefan Roese <sr@denx.de>