firmware/raspberrypi: Add two new messages

RPI_FIRMWARE_GET_CLOCK_MEASURED is similar to RPI_FIRMWARE_GET_CLOCK,
except that it uses a hardware feature to count the clock pulses to
measure the real clock speed.

RPI_FIRMWARE_NOTIFY_REBOOT informs the firmware that the OS is about to
reboot, allowing it to make any necessary adjustments.

Signed-off-by: Phil Elwell <phil@raspberrypi.org>