feat(pm_low_power_modes): Note that RT Linux does not support LPM

Since this page shows in RT Linux documentation, denote that no low
power modes are supported in RT Linux.

Signed-off-by: Kendall Willis <k-willis@ti.com>

Author: kwillis01

source/linux/Foundational_Components/Power_Management/pm_low_power_modes.rst

@@ -393,6 +393,8 @@ Limitations
       to anything. When USB is connected and a low power mode is entered, the
       EVM fails to resume to an active Linux state.
 
+RT (Real-Time) Linux does not support any low power modes.
+
 HWRNG support on GP devices is incompatible with Deep Sleep and MCU Only
 modes. To test LPM on GP devices, HWRNG has to be unloaded one-time
 before running the Suspend-to-RAM command: