util/intelmetool: Fix warning building with 32-bit

util/intelmetool: Fix warning building with 32-bit

On a 32-bit system, pointers are 32-bit wide, and not 64-bit, resulting
in the warning below.

```
mmap.c: In function ‘map_physical_exact’:
mmap.c:26:20: warning: cast to pointer from integer of different size [-Wint-to-pointer-cast]
   virt_addr = mmap((void*)mapto, len, PROT_WRITE | PROT_READ,
                    ^
```

Fix this by using compatible types.

Change-Id: I4ede26127efcbd5668b978e6880a0535607e373d
Signed-off-by: Paul Menzel <paulepanter@users.sourceforge.net>
Reviewed-on: https://review.coreboot.org/17970
Tested-by: build bot (Jenkins)
Reviewed-by: Nico Huber <nico.h@gmx.de>
Reviewed-by: Martin Roth <martinroth@google.com>