Add the bdev_type attribute to the rpmb_device type

This patch fixes the following SELinux denial, reported by Treehugger
for patch https://android-review.git.corp.google.com/c/platform/system/apex/+/1782069:

07-29 02:52:54.320   582   582 I auditd  : type=1400 audit(0.0:4): avc: denied { getattr } for comm="apexd" path="/dev/block/mmcblk0rpmb" dev="tmpfs" ino=15991 scontext=u:r:apexd:s0

Bug: 194450129
Test: Untested.
Change-Id: I5b6cd8377cfef6d28ec2c2f3f63c68e2530c5ab4
Signed-off-by: Bart Van Assche <bvanassche@google.com>