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

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 <[email protected]>
Change-Id: I16797e7256c7eb699a7b8846356fe430d0fe0aa1