Change mbed- in mbed-arm-an547-mps3-startup to mdh-

An application that uses MCU-Driver-HAL can decide to either use the
default startup and linker files by linking `<board>-startup`, or
provide its own using `target_sources()` for startup file and
`target_link_options()` for linker script.

The Open IoT SDK uses the default startup and linker files for some of
its example, and we want all names referenced in the SDK to be without
"mbed". Use the prefix "mdh-" instead.

Signed-off-by: Lingkai Dong <lingkai.dong@arm.com>