docs: phy: place documentation under driver-api

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

Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>
diff --git a/Documentation/driver-api/index.rst b/Documentation/driver-api/index.rst
index cf39b8f..eff22db 100644
--- a/Documentation/driver-api/index.rst
+++ b/Documentation/driver-api/index.rst
@@ -85,6 +85,7 @@
    parport-lowlevel
    pps
    ptp
+   phy/index
    pti_intel_mid
    pwm
    rfkill