powerpc/64: mark start_here_multiplatform as __ref

Otherwise, the following warning is encountered:

WARNING: vmlinux.o(.text+0x3dc6): Section mismatch in reference from the variable start_here_multiplatform to the function .init.text:.early_setup()
The function start_here_multiplatform() references
the function __init .early_setup().
This is often because start_here_multiplatform lacks a __init
annotation or the annotation of .early_setup is wrong.

Fixes: 56c46bba ("powerpc/64: Fix booting large kernels with STRICT_KERNEL_RWX")
Cc: Russell Currey <ruscur@russell.cc>
Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/kernel/head_64.S

@@ -904,6 +904,7 @@ p_toc:	.8byte	__toc_start + 0x8000 - 0b
 /*
  * This is where the main kernel code starts.
  */
+__REF
 start_here_multiplatform:
 	/* set up the TOC */
 	bl      relative_toc
@@ -979,6 +980,7 @@ start_here_multiplatform:
 	RFI
 	b	.	/* prevent speculative execution */
 
+	.previous
 	/* This is where all platforms converge execution */
 
 start_here_common: