clk: ep93xx: add DT support for Cirrus EP93xx

Rewrite EP93xx clock driver located in arch/arm/mach-ep93xx/clock.c
trying to do everything the device tree way:

- provide clock acces via of
- drop clk_hw_register_clkdev
- drop init code and use module_auxiliary_driver

Co-developed-by: Alexander Sverdlin <alexander.sverdlin@gmail.com>
Signed-off-by: Alexander Sverdlin <alexander.sverdlin@gmail.com>
Signed-off-by: Nikita Shubin <nikita.shubin@maquefel.me>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>