[imu] aspirin_2_spi: default AHRS_PROPAGATE_FREQUENCY is 512

With DLPF_256HZ (value zero) the internal sampling rate is 8kHz and with a sample rate divider of 1
the output freq was actually 4kHz and not 500Hz. Since the data is polled from the mpu at PERIODIC_FREQUENCY
the AHRS_PROPAGATE_FREQUENCY (defaulting to PERIODIC_FREQUENCY) is also 512Hz for a default rotorcraft config.

Set the output freq to 2kHz now, AHRS_PROPAGATE_FREQUENCY stays at PERIODIC_FREQUENCY=512Hz so doesn't have to be
defined in the airframe file.

Also added defaults for PERIODIC_FREQUENCY of 60 and 120Hz:
- DLPF_42HZ
- output rate of 100Hz
In these cases add <define name="AHRS_PROPAGATE_FREQUENCY" value="100"/>