fix(cpsw): Clarify switch mode requirements and remove QSGMII limitation

Replace incorrect QSGMII-only restriction with the explaination that
switch mode requires two external ethernet ports and works independently
of MAC interface modes.

This corrects misleading documentation that could confuse users about
switch mode capabilities across different interface configurations.

Signed-off-by: Chintan Vankar <c-vankar@ti.com>

Author: chintanv133

source/linux/Foundational_Components/Kernel/Kernel_Drivers/Network/CPSWng-Native-Ethernet.rst

@@ -84,7 +84,10 @@ for utilizing the MAC mode features.
 Switch Mode
 """""""""""
 
-All the SoCs listed above support Switch Mode when configured in QSGMII mode.
+.. note::
+
+   Switch mode functionality requires at least two external ethernet ports to work, it is
+   independent of the MAC interface modes.
 
 The Switch mode can be enabled by configuring devlink driver parameter
 "switch_mode" to 1/true::