xtensa: processor.h: add init_arch() prototype

Add the prototype for init_arch() to asm/processor.h to prevent a
build warning:

arch/xtensa/kernel/setup.c:244:13: warning: no previous prototype for 'init_arch' [-Wmissing-prototypes]
  244 | void __init init_arch(bp_tag_t *bp_start)

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: Chris Zankel <chris@zankel.net>
Cc: Max Filippov <jcmvbkbc@gmail.com>
Message-Id: <20230920052139.10570-6-rdunlap@infradead.org>
Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>