Change name of Marvell Armada38x RTC driver

Two modules with the same name cannot be loaded, so Marvell specific drivers
cannot have the same name as generic drivers.
Files with the same name, even in different folder overlaps their .o files.
Change armada38x/rtc.c to armada38x/armada38x_rtc.c fix it.
Preparation for adding this driver to GENERIC config for ARMv7
Marvell platforms.

Submitted by: Rafal Kozik <rk@semihalf.com>
Obtained from: Semihalf
Sponsored by: Semihalf
Differential Revision: https://reviews.freebsd.org/D12185