Add global mpcore timer node to Armada 38x DTS

Changes:

• global mpcore timer dts node added
• required by driver 'clock-frequency' property added

Reviewed by: ian
Obtained from: Semihalf
Sponsored by: Stormshield
Submitted by: Bartosz Szczepanek <bsz@semihalf.com>
Differential revision: https://reviews.freebsd.org/D4213