ARM: ep93xx: normalize clk API

It's a combination of the patch from Arnd Bergmann, which added empty stubs
for clk_round_rate() and clk_set_parent() and a working trivial
implementation of clk_get_parent(). The later is required for ADC driver.

Signed-off-by: Alexander Sverdlin <alexander.sverdlin@gmail.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>