ARM: memremap: implement arch_memremap_wb()

The generic memremap() falls back to using ioremap_cache() to create
MEMREMAP_WB mappings if the requested region is not already covered
by the linear mapping, unless the architecture provides an implementation
of arch_memremap_wb().

Since ioremap_cache() is not appropriate on ARM to map memory with the
same attributes used for the linear mapping, implement arch_memremap_wb()
which does exactly that. Also, relax the WARN() check to allow MT_MEMORY_RW
mappings of pfn_valid() pages.

Cc: Russell King <rmk+kernel@arm.linux.org.uk>
Acked-by: Dan Williams <dan.j.williams@intel.com>
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>

arch/arm/include/asm/io.h

@@ -392,6 +392,9 @@ void __iomem *ioremap(resource_size_t res_cookie, size_t size);
 #define ioremap ioremap
 #define ioremap_nocache ioremap
 
+/*
+ * Do not use ioremap_cache for mapping memory. Use memremap instead.
+ */
 void __iomem *ioremap_cache(resource_size_t res_cookie, size_t size);
 #define ioremap_cache ioremap_cache
 
@@ -408,6 +411,9 @@ void __iomem *ioremap_wc(resource_size_t res_cookie, size_t size);
 void iounmap(volatile void __iomem *iomem_cookie);
 #define iounmap iounmap
 
+void *arch_memremap_wb(phys_addr_t phys_addr, size_t size);
+#define arch_memremap_wb arch_memremap_wb
+
 /*
  * io{read,write}{16,32}be() macros
  */