riscv: mm: init: Pass a pointer to virt_to_page()

Functions that work on a pointer to virtual memory such as
virt_to_pfn() and users of that function such as
virt_to_page() are supposed to pass a pointer to virtual
memory, ideally a (void *) or other pointer. However since
many architectures implement virt_to_pfn() as a macro,
this function becomes polymorphic and accepts both a
(unsigned long) and a (void *).

Fix this in the RISCV mm init code, so we can implement
a strongly typed virt_to_pfn().

Reviewed-by: Alexandre Ghiti <alexghiti@rivosinc.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>