Fix 'make headers_check' on sh64

Cleanup for user headers, as noted:

asm-sh64/page.h requires asm-generic/memory_model.h, which does not exist in exported headers
asm-sh64/shmparam.h requires asm/cache.h, which does not exist in exported headers
asm-sh64/signal.h requires asm/processor.h, which does not exist in exported headers
asm-sh64/user.h requires asm/processor.h, which does not exist in exported headers

Signed-off-by: Paul Mundt <lethal@linux-sh.org>
Signed-off-by: David Woodhouse <dwmw2@infradead.org>