boards: arm: mps2_an385: correct flash and SRAM sizes

Add a note in the board documentation regarding the
correct size of the available flash and sram memory.

Signed-off-by: Ioannis Glaropoulos <Ioannis.Glaropoulos@nordicsemi.no>

boards/arm/mps2_an385/doc/index.rst

@@ -41,7 +41,7 @@ ARM V2M MPS2 provides the following hardware components:
 - ARM Cortex-M3 (AN385)
 - ARM IoT Subsystem for Cortex-M
 - Form factor: 140x120cm
-- SRAM: 8MB single cycle SRAM, 16MB PSRAM
+- ZBTSRAM: 8MB single cycle SRAM, 16MB PSRAM
 - Video: QSVGA touch screen panel, 4bit RGB VGA connector
 - Audio: Audio Codec
 - Debug:
@@ -57,6 +57,9 @@ ARM V2M MPS2 provides the following hardware components:
   - GPIO
   - SPI
 
+.. note::
+   4 MB of flash memory (in ZBTSRAM 1, starting at address 0x00400000) and 4 MB of RAM
+   (in ZBTSRAM 2 & 3, starting at address 0x20000000) are available.
 
 Supported Features
 ==================