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),