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>