mmc: core: Add helper for parsing clock phase properties

mmc: core: Add helper for parsing clock phase properties

Drivers for MMC hosts that accept phase corrections can take advantage
of the helper by embedding an instance of struct mmc_clk_phase_map in
their private data and invoking mmc_of_parse_clk_phase() to extract
phase parameters. It is the responsibility of the host driver to
translate and apply the extracted values to hardware as required.

Signed-off-by: Andrew Jeffery <andrew@aj.id.au>
Link: https://lore.kernel.org/r/20210114031433.2388532-2-andrew@aj.id.au
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>