Fix bit assignment in PL310_POWER_CTRL

Align to ARM specification:
http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.ddi0246f/BEIEHICF.html

Submitted by: Marcin Wojtas <mw@semihalf.com>
Obtained from: Semihalf
Sponsored by: Stormshield
Reviewed by: meloun-miracle-cz
Differential revision: https://reviews.freebsd.org/D10223