mm/ARM: fix stale comment about VALID_PAGE()

VALID_PAGE() has been removed from kernel long time ago,
so fix the comment.

Signed-off-by: Jiang Liu <jiang.liu@huawei.com>
Cc: Russell King <linux@arm.linux.org.uk>
Cc: Will Deacon <will.deacon@arm.com>
Cc: Nicolas Pitre <nico@linaro.org>
Cc: Stephen Boyd <sboyd@codeaurora.org>
Cc: Giancarlo Asnaghi <giancarlo.asnaghi@st.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

arch/arm/include/asm/memory.h

@@ -276,12 +276,6 @@ static inline __deprecated void *bus_to_virt(unsigned long x)
 /*
  * Conversion between a struct page and a physical address.
  *
- * Note: when converting an unknown physical address to a
- * struct page, the resulting pointer must be validated
- * using VALID_PAGE().  It must return an invalid struct page
- * for any physical address not corresponding to a system
- * RAM address.
- *
  *  page_to_pfn(page)	convert a struct page * to a PFN number
  *  pfn_to_page(pfn)	convert a _valid_ PFN number to struct page *
  *