pageblock_order can be (at least) an unsigned int or an unsigned long
depending on the kernel config and architecture, so use max_t(unsigned
long ...) when comparing it.

fixes these warnings:

In file included from include/linux/list.h:8:0,
                 from include/linux/kobject.h:20,
                 from include/linux/of.h:21,
                 from drivers/of/of_reserved_mem.c:17:
drivers/of/of_reserved_mem.c: In function ‘__reserved_mem_alloc_size’:
include/linux/kernel.h:748:17: warning: comparison of distinct pointer types lacks a cast
  (void) (&_max1 == &_max2);  \
                 ^
include/linux/kernel.h:747:9: note: in definition of macro ‘max’
  typeof(y) _max2 = (y);   \
         ^
drivers/of/of_reserved_mem.c:131:48: note: in expansion of macro ‘max’
   align = max(align, (phys_addr_t)PAGE_SIZE << max(MAX_ORDER - 1, pageblock_ord
                                                ^
include/linux/kernel.h:748:17: warning: comparison of distinct pointer types lacks a cast
  (void) (&_max1 == &_max2);  \
                 ^
include/linux/kernel.h:747:21: note: in definition of macro ‘max’
  typeof(y) _max2 = (y);   \
                     ^
drivers/of/of_reserved_mem.c:131:48: note: in expansion of macro ‘max’
   align = max(align, (phys_addr_t)PAGE_SIZE << max(MAX_ORDER - 1, pageblock_ord
                                                ^

Fixes: 1cc8e345

 ("drivers: of: of_reserved_mem: fixup the alignment with CMA setup")
Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Rob Herring <robh@kernel.org>