csky: Cast argument to virt_to_pfn() to (void *)

The virt_to_pfn() function takes a (void *) as argument, fix
this up to avoid exploiting the unintended polymorphism of
virt_to_pfn.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Guo Ren <guoren@linux.alibaba.com>
Signed-off-by: Guo Ren <guoren@kernel.org>