mmc: core: Move mmc_of_parse_voltage() to host.c

MMC OF parsing functions, which parses various host DT properties, should
stay close to each other. Therefore, let's move mmc_of_parse_voltage()
close to mmc_of_parse() into host.c.

Additionally, there is no reason to build the code only when CONFIG_OF is
set, as there should be stub functions for the OF helpers that is being
used, so let's drop this condition as well.

Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>