Documentation: Document esp32p4-function-ev-board:psram_usrheap

This commit adds an entry for the ESP32-P4-Function-EV-Board's
`psram_usrheap` defconfig on docs.

Signed-off-by: Tiago Medicci Serrano <tiago.medicci@espressif.com>

Documentation/platforms/risc-v/esp32p4/boards/esp32p4-function-ev-board/index.rst

@@ -295,6 +295,17 @@ To save power without using sleep modes, lowering the clock speed is another app
 ``CONFIG_ESPRESSIF_DFS`` option needs to enabled and minimum CPU frequency needs to set under ``CONFIG_ESPRESSIF_MIN_CPU_FREQ`` option.
 With these options, the device scales the CPU clock according to workload.
 
+psram_usrheap
+-------------
+
+This configuration enables allocating the userspace heap into SPIRAM and reserves the
+internal RAM for kernel heap. For instance, for a 32MB PSRAM::
+
+    nsh> free
+          total       used       free    maxused    maxfree  nused  nfree name
+        602004       6492     595512       6872     595512     36      1 Kmem
+      33554428       4276   33550152       4656   33550152      8      1 Umem
+
 pwm
 ---
 

Documentation/platforms/risc-v/esp32p4/index.rst

@@ -348,6 +348,7 @@ Parallel IO        No
 LCD Interface      No
 MIPI DSI           No
 Timers             Yes
+SPIRAM / PSRAM     Yes
 Watchdog           Yes     MWDT0/1 and RWDT
 Ethernet           No
 Brownout           No