target/arm: Move get_phys_addr to ptw.c

target/arm: Move get_phys_addr to ptw.c

Begin moving all of the page table walking functions
out of helper.c, starting with get_phys_addr().

Create a temporary header file, "ptw.h", in which to
share declarations between the two C files while we
are moving functions.

Move a few declarations to "internals.h", which will
remain used by multiple C files.

Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20220604040607.269301-3-richard.henderson@linaro.org
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>