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

docs/plat/xilinx-versal.rst

@@ -14,7 +14,7 @@ To build:
 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
 ```

plat/xilinx/versal/platform.mk

@@ -31,7 +31,7 @@ ifdef VERSAL_BL32_MEM_BASE
     $(eval $(call add_define,VERSAL_BL32_MEM_SIZE))
 endif
 
-VERSAL_PLATFORM ?= versal_virt
+VERSAL_PLATFORM ?= silicon
 $(eval $(call add_define_val,VERSAL_PLATFORM,VERSAL_PLATFORM_ID_${VERSAL_PLATFORM}))
 
 VERSAL_CONSOLE	?=	pl011