arm-trusted-firmware-mediatek: use MT/s as DDR speed unit

arm-trusted-firmware-mediatek: use MT/s as DDR speed unit

Usually we do not use MHz to describe the DDR transmission rate. In
fact, the clock frequency of the DDR3-1866 is only 933 MHz. MT/s is
a more commonly used unit.

Signed-off-by: Shiji Yang <yangshiji66@outlook.com>