sys/sys_io.h: Memory mapped I/O APIs must be pointer-sized, not 32 bit!

The mm_reg_t type used to represent a MMIO address (e.g. in the
sys_write*() APIs) was defined to a uint32_t, which is obviously wrong
on 64 bit systems where devices can be mapped anywhere.

Signed-off-by: Andy Ross <andrew.j.ross@intel.com>