xtensa: provide default platform_pcibios_init implementation

This fixes the following build error:

  arch/xtensa/kernel/built-in.o:(.init.literal+0xe8): undefined reference
	to `platform_pcibios_init'
  arch/xtensa/kernel/built-in.o: In function `setup_arch':
  (.init.text+0x20e): undefined reference to `platform_pcibios_init'

and allows platform to omit definition of platform_pcibios_init if it's
empty.
Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>
Signed-off-by: Chris Zankel <chris@zankel.net>

arch/xtensa/kernel/platform.c

@@ -36,6 +36,7 @@ _F(void, power_off, (void), { while(1); });
 _F(void, idle, (void), { __asm__ __volatile__ ("waiti 0" ::: "memory"); });
 _F(void, heartbeat, (void), { });
 _F(int,  pcibios_fixup, (void), { return 0; });
+_F(void, pcibios_init, (void), { });
 
 #ifdef CONFIG_XTENSA_CALIBRATE_CCOUNT
 _F(void, calibrate_ccount, (void),

arch/xtensa/platforms/iss/setup.c

@@ -38,12 +38,6 @@ void __init platform_init(bp_tag_t* bootparam)
 
 }
 
-#ifdef CONFIG_PCI
-void platform_pcibios_init(void)
-{
-}
-#endif
-
 void platform_halt(void)
 {
 	pr_info(" ** Called platform_halt() **\n");