feat(fvp): allow configurable FVP Trusted SRAM size

In some build configurations TF-A can exceed the existing 256KB SRAM,
triggering a build failure. More recent versions of the base FVP allow
you to configure a larger Trusted SRAM of 512KB.

This change introduces the `FVP_TRUSTED_SRAM_SIZE` build option, which
allows you to explicitly specify how much of the Trusted SRAM to
utilise, e.g.:

    FVP_TRUSTED_SRAM_SIZE=384

This allows previously-failing configurations to build successfully by
utilising more than the originally-allocated 256KB of the Trusted SRAM
while maintaining compatibility with older configurations/models that
only require/have 256KB.

Change-Id: I8344d3718564cd2bd53f1e6860e2fe341ae240b0
Signed-off-by: Chris Kay <chris.kay@arm.com>
diff --git a/plat/arm/board/fvp/include/platform_def.h b/plat/arm/board/fvp/include/platform_def.h
index 9e72ba0..cd468e1 100644
--- a/plat/arm/board/fvp/include/platform_def.h
+++ b/plat/arm/board/fvp/include/platform_def.h
@@ -43,7 +43,7 @@
  */
 #define PLAT_ARM_CLUSTER_COUNT		U(FVP_CLUSTER_COUNT)
 
-#define PLAT_ARM_TRUSTED_SRAM_SIZE	UL(0x00040000)	/* 256 KB */
+#define PLAT_ARM_TRUSTED_SRAM_SIZE	(FVP_TRUSTED_SRAM_SIZE * UL(1024))
 
 #define PLAT_ARM_TRUSTED_ROM_BASE	UL(0x00000000)
 #define PLAT_ARM_TRUSTED_ROM_SIZE	UL(0x04000000)	/* 64 MB */