docs(linux): AM64x: Update CPSW documentation

CPSW interface associated with DP83869 PHY in AM642-EVM needs to be
brought up manually. Update the documentation to mention the same.

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

Author: chintanv133

source/linux/Foundational_Components/Kernel/Kernel_Drivers/Network/_K3-CPSW-common.rst

@@ -12,6 +12,14 @@ The network interface can be configured automatically depending on root file sys
 
    ifconfig eth0 <ip> netmask <mask> up
 
+.. ifconfig:: CONFIG_part_variant in ('AM64X')
+
+   .. note::
+
+      The DP83869 PHY (associated with the eth1 CPSW interface for AM642-EVM) is
+      not built into the kernel image. It is not available during early boot as
+      a result, and must be brought up using the commands provided above.
+
 .. rubric:: Get driver information
    :name: k3-ethtool-i-driver