[powersave] serial/uart (used for bluetooth) runtime PM missing (reverted)

According to /sys/kernel/debug/pm_genpd/pm_genpd_summary, one of the things holding up CX rail suspend was the bluetooth UART (a98000.serial on x1e80100).

Runtime PM for that driver was reverted in a699213d4e due to regressions, but a fix is already available. Looks like Linaro is already aware :) but tracking this here as well.