docs: phy: place documentation under driver-api

This subsystem-specific documentation belongs to the
driver-api.

Signed-off-by: Mauro Carvalho Chehab <[e-mail]>

Documentation/devicetree/bindings/phy/phy-bindings.txt

···
 This document explains only the device tree data binding. For general
-information about PHY subsystem refer to Documentation/phy.txt
+information about PHY subsystem refer to Documentation/driver-api/phy/phy.rst
 
 PHY device node
 ===============

Documentation/devicetree/bindings/phy/phy-pxa-usb.txt

···
 	};
 
 This document explains the device tree binding. For general
-information about PHY subsystem refer to Documentation/phy.txt
+information about PHY subsystem refer to Documentation/driver-api/phy/phy.rst

Documentation/driver-api/index.rst

···
    parport-lowlevel
    pps
    ptp
+   phy/index
    pti_intel_mid
    pwm
    rfkill

Documentation/driver-api/phy/index.rst

···
+=====================
+Generic PHY Framework
+=====================
+
+.. toctree::
+
+   phy
+   samsung-usb2
+
+.. only::  subproject and html
+
+   Indices
+   =======
+
+   * :ref:`genindex`
+

Documentation/index.rst

···
    usb/index
    misc-devices/index
    mic/index
-   phy/samsung-usb2
    scheduler/index
 
 Architecture-specific documentation

MAINTAINERS

···
 L:	[e-mail]
 S:	Supported
 F:	Documentation/devicetree/bindings/phy/samsung-phy.txt
-F:	Documentation/phy/samsung-usb2.rst
+F:	Documentation/driver-api/phy/samsung-usb2.rst
 F:	drivers/phy/samsung/phy-exynos4210-usb2.c
 F:	drivers/phy/samsung/phy-exynos4x12-usb2.c
 F:	drivers/phy/samsung/phy-exynos5250-usb2.c