mmc: sdhci-of-dwcmshc: factor out code for th1520_init()

mmc: sdhci-of-dwcmshc: factor out code for th1520_init()

Different socs have initialization operations in
the probe process, which are summarized as functions.

This patch first factor out init function for th1520.

Signed-off-by: Chen Wang <unicorn_wang@outlook.com>
Reviewed-by: Drew Fustini <drew@pdp7.com>
Tested-by: Drew Fustini <drew@pdp7.com> # TH1520
Tested-by: Inochi Amaoto <inochiama@outlook.com> # Duo and Huashan Pi
Acked-by: Adrian Hunter <adrian.hunter@intel.com>
Link: https://lore.kernel.org/r/23c6a81052a6dd3660d60348731229d60a209b32.1722847198.git.unicorn_wang@outlook.com
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>