Split get_sar_value function for Marvell ArmadaXP and Armada38X

get_sar_value is implemented only for ArmadaXP and Armada38X. Splitting it for
two different functions and change registers names result in more generic code.

Submitted by: Rafal Kozik <rk@semihalf.com>
Reviewed by: andrew
Obtained from: Semihalf
Sponsored by: Stormshield
Differential Revision: https://reviews.freebsd.org/D14736