add vchiq mailbox unbind to pieeprom-update.sh #3289

Job	Run time
Yocto / Approved commit	4s
Yocto / Source mirror IAM role	4s
Yocto / Device info	47s
Yocto / Build	27s
Yocto / Deploy AMI	1s
Yocto / Test	1s
Yocto / Deploy to S3	1s
Yocto / All jobs (revpi-connect-4, balena-cloud.com)	3s
Yocto / Deploy hostApp	1s
	1m 29s

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