docs: plat: stm32mp1: Document the usage of DTB_FILE_NAME variable

Since STM32MP1 platform supports different boards, it is necessary
to build for a particular board. With the current instructions, the
user has to modify the DTB_FILE_NAME variable in platform.mk for
building for a particular board, but this can be avoided by passing
the appropriate board DTB name via DTB_FILE_NAME make variable.
Hence document the same in platform doc.

Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Change-Id: I16797e7256c7eb699a7b8846356fe430d0fe0aa1

docs/plat/stm32mp1.rst

@@ -81,7 +81,7 @@ To build with SP_min:
 
 .. code:: bash
 
-    make CROSS_COMPILE=arm-linux-gnueabihf- PLAT=stm32mp1 ARCH=aarch32 ARM_ARCH_MAJOR=7 AARCH32_SP=sp_min
+    make CROSS_COMPILE=arm-linux-gnueabihf- PLAT=stm32mp1 ARCH=aarch32 ARM_ARCH_MAJOR=7 AARCH32_SP=sp_min DTB_FILE_NAME=stm32mp157c-ev1.dtb
     cd <u-boot_directory>
     make stm32mp15_basic_defconfig
     make DEVICE_TREE=stm32mp157c-ev1 all