riscv: fix the global name pfn_base confliction error

RISCV uses a global variable pfn_base for page/pfn translation. But this
is a common name and will be used elsewhere. In those cases, the
page-pfn macros which refer to this name will be referred to the
local/input variable instead. (such as in vfio_pin_pages_remote). This
make everything wrong.

This patch changes the name from pfn_base to riscv_pfn_base to fix
this problem.

Signed-off-by: Kenneth Lee <liguozhu@hisilicon.com>
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>