plat: xilinx: versal: Make silicon default build target

This patch makes default build target as silicon instead of QEMU.
The default can be overwritten by specifying it through build flag
VERSAL_PLATFORM.

Signed-off-by: Siva Durga Prasad Paladugu <siva.durga.paladugu@xilinx.com>
Signed-off-by: Jolly Shah <jolly.shah@xilinx.com>
Change-Id: Ia4cb1df1f206db3e514e8ce969acca875e973ace
diff --git a/docs/plat/xilinx-versal.rst b/docs/plat/xilinx-versal.rst
index 231286e..95c89a8 100644
--- a/docs/plat/xilinx-versal.rst
+++ b/docs/plat/xilinx-versal.rst
@@ -14,7 +14,7 @@
 make RESET_TO_BL31=1 CROSS_COMPILE=aarch64-none-elf- PLAT=versal bl31
 ```
 
-To build ATF for different platform (for now its just versal virtual "versal_virt")
+To build ATF for different platform (supported are "silicon"(default) and "versal_virt")
 ```bash
 make RESET_TO_BL31=1 CROSS_COMPILE=aarch64-none-elf- PLAT=versal VERSAL_PLATFORM=versal_virt bl31
 ```