riscv: keep 32-bit kernel to 32-bit phys_addr_t

While rv32 technically has 34-bit physical addresses, no current platforms
use it and it's likely to shake out driver bugs.

Let's keep 64-bit phys_addr_t off on 32-bit builds until one shows up,
since other work will be needed to make such a system useful anyway.

PHYS_ADDR_T_64BIT is def_bool 64BIT, so just remove the select.

Signed-off-by: Olof Johansson <olof@lixom.net>
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>