Make get_tclk and get_cpu_freq generic for Marvell armv7 SoCs

In GENERIC kernel choosing proper get_tclk and get_cpu_freq implementation must
be done in runtime. Kernel for both SoC need to have implementation of each
other functions, so common file list mv/files.arm7 is added.
Marvell armv5 SoC have their own non-generic implementation of those function.

Submitted by: Rafal Kozik <rk@semihalf.com>
Obtained from: Semihalf
Sponsored by: Stormshield
Differential Revision: https://reviews.freebsd.org/D14739