riscv: Fix implicit declaration of 'page_to_section'

With CONFIG_SPARSEMEM and !CONFIG_SPARSEMEM_VMEMMAP,

arch/riscv/include/asm/pgtable.h: In function ‘mk_pte’:
include/asm-generic/memory_model.h:64:14: error: implicit declaration of function ‘page_to_section’; did you mean ‘present_section’? [-Werror=implicit-function-declaration]
  int __sec = page_to_section(__pg);   \
              ^~~~~~~~~~~~~~~

Fixed by changing mk_pte() from inline function to macro.

Cc: Logan Gunthorpe <logang@deltatee.com>
Fixes: d95f1a54

 ("RISC-V: Implement sparsemem")
Signed-off-by: Kefeng Wang <wangkefeng.wang@huawei.com>
[paul.walmsley@sifive.com: fixed checkpatch errors]
Signed-off-by: Paul Walmsley <paul.walmsley@sifive.com>