bus: vexpress-config: Annotate struct vexpress_syscfg_func with __counted_by

Prepare for the coming implementation by GCC and Clang of the __counted_by
attribute. Flexible array members annotated with __counted_by can have
their accesses bounds-checked at run-time checking via CONFIG_UBSAN_BOUNDS
(for array indexing) and CONFIG_FORTIFY_SOURCE (for strcpy/memcpy-family
functions).

As found with Coccinelle[1], add __counted_by for struct vexpress_syscfg_func.

[1] https://github.com/kees/kernel-tools/blob/trunk/coccinelle/examples/counted_by.cocci

Cc: Liviu Dudau <liviu.dudau@arm.com>
Cc: Sudeep Holla <sudeep.holla@arm.com>
Cc: Lorenzo Pieralisi <lpieralisi@kernel.org>
Cc: Rob Herring <robh@kernel.org>
Cc: Yangtao Li <frank.li@vivo.com>
Cc: linux-arm-kernel@lists.infradead.org
Signed-off-by: Kees Cook <keescook@chromium.org>
Reviewed-by: "Gustavo A. R. Silva" <gustavoars@kernel.org>
Link: https://lore.kernel.org/r/20230922175234.work.134-kees@kernel.orgSigned-off-by: Sudeep Holla <sudeep.holla@arm.com>

drivers/bus/vexpress-config.c

@@ -54,7 +54,7 @@ struct vexpress_syscfg_func {
 	struct vexpress_syscfg *syscfg;
 	struct regmap *regmap;
 	int num_templates;
-	u32 template[]; /* Keep it last! */
+	u32 template[] __counted_by(num_templates); /* Keep it last! */
 };
 
 struct vexpress_config_bridge_ops {