scmi: Implement scmi_clknode_recalc_freq method

Allow the SCMI clock frequency to be queried back, useful for testing
the IRQ path via sysctl access.

Reviewed by: andrew
Sponsored by: Arm Ltd
Differential Revision: https://reviews.freebsd.org/D43038

(cherry picked from commit ecd8cc84dcee7d08be8663f721f0502dc5b65808)