spi.h: add doc to make sure people free the returned buffer after maa_spi_write

Signed-off-by: Brendan Le Foll <brendan.le.foll@intel.com>
maa.i: add typemaps for uint8_t and uint8_t*

Signed-off-by: Brendan Le Foll <brendan.le.foll@intel.com>