arc: fix missing _firq_stack symbol when INIT_STACKS=y and NUM_BANKS=1

There is no FIRQ stack in the system in this case, so do not initialize
it.

Change-Id: I8bc068ce43ac8a39909994d8cc01ba0c6a17f4ae
Signed-off-by: Benjamin Walsh <[email protected]>

Author: benwrs

arch/arc/core/reset.S

@@ -87,12 +87,15 @@ SECTION_FUNC(TEXT,__start)
 	mov_s r2, CONFIG_ISR_STACK_SIZE
 	jl memset
 
+#if CONFIG_RGF_NUM_BANKS != 1
 	mov_s r0, _firq_stack
 	mov_s r1, 0xaa
 	mov_s r2, CONFIG_FIRQ_STACK_SIZE
 	jl memset
 #endif
 
+#endif /* CONFIG_INIT_STACKS */
+
 	mov sp, INIT_STACK
 	add sp, sp, INIT_STACK_SIZE