powerpc: Print linux_banner in prom_init

So at least you can see what kernel you're booting if you die
before the kernel prints it mid-way through start_kernel().

Signed-off-by: Michael Ellerman <michael@ellerman.id.au>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>

arch/powerpc/kernel/prom_init.c

@@ -2283,6 +2283,8 @@ unsigned long __init prom_init(unsigned long r3, unsigned long r4,
 	 */
 	prom_init_stdout();
 
+	prom_printf("Preparing to boot %s", PTRRELOC((char *)linux_banner));
+
 	/*
 	 * Get default machine type. At this point, we do not differentiate
 	 * between pSeries SMP and pSeries LPAR