arch/arm/armv7: Make null dcache_apply_all macro for userspace

Make an empty macro for dcache_apply_all for code running in userspace
so that we don't hang the system.

BUG=b:161554141
TEST=Run board through a bunch of recovery cycles.

Signed-off-by: Martin Roth <martin@coreboot.org>
Change-Id: I3dc0f40dfe4d4a699528068154eee2d3c23d3d74
Reviewed-on: https://review.coreboot.org/c/coreboot/+/43787
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Julius Werner <jwerner@chromium.org>
Reviewed-by: Furquan Shaikh <furquan@google.com>

src/arch/arm/armv7/cpu.S

@@ -23,6 +23,19 @@
  * THIS FUNCTION MUST PRESERVE THE VALUE OF r10
  */
 
+#if ENV_USER_SPACE
+/*
+ * Empty macro for code running in userspace.  Trying to manipulate the
+ * cache from userspace hangs the system. To run code at a privileged level,
+ * the userspace code needs to execute an API call to the privileged mode
+ * code.
+ */
+.macro	dcache_apply_all crm
+	bx	lr
+.endm
+
+#else
+
 .macro	dcache_apply_all crm
 	dsb
 	mov	r3, #-2			@ initialize level so that we start at 0
@@ -82,6 +95,8 @@
 	bx	lr
 .endm
 
+#endif /* ENV_USER_SPACE */
+
 /*
  * Bring an ARM processor we just gained control of (e.g. from IROM) into a
  * known state regarding caches/SCTLR. Completely cleans and invalidates