docs(psci): add a mention to the pwr_domain_pwr_down_wfi()

The function got renamed to pwr_domain_pwr_down() but have a reference
to it for anyone wondering where it went.

Change-Id: Ica5fa11b9f18a7446c188e37b9f1d5508f4cf749
Signed-off-by: Boyan Karatotev <boyan.karatotev@arm.com>
1 file changed