docs(linux): J721E: Fix the documentation for USB

Currently same USB documentation is used for all the J7 family devices.
But this documentation is only valid for J721E EVM. Fix that by making
documentation specific to the J721E EVM.

Signed-off-by: Hrushikesh Salunke <h-salunke@ti.com>

Author: hrushikesh221

source/linux/Foundational_Components/Kernel/Kernel_Drivers/USB/CDNS3.rst

@@ -19,7 +19,7 @@ The USB subsystem is integrated in TI |__PART_FAMILY_DEVICE_NAMES__| devices.
 EVM
 ***
 
-.. ifconfig:: CONFIG_part_family in ('J7_family')
+.. ifconfig:: CONFIG_part_variant in ('J721E')
 
     The following diagram depicts USB integration in J721E EVM.