Bitnami wordpress EC2 AMI:

Default User: "User"
PWD: check from ec2 instance-Settings->System Log

Mysql Access:

USer: root
PWD: ramdom generated password: check from ec2 instance-Settings->System Log

PhpMyadmin for Bitnami:

From your PC, by which you trying to connect the AWS server:

$ ssh -N -L 8888:127.0.0.1:80 -i bitnami-hosting.pem bitnami@xyz.bitnamiapp.com

to access phpmyadmin:
http://127.0.0.1/phpmyadmin/

user name: root
pwd: random generated password (check from ec2 instance-Settings->System Log)

reference:
https://wiki.bitnami.com/Components/phpMyAdmin