mmc: atmel-mci: move atmel MCI header file

Move the contents of linux/atmel-mci.h into
drivers/mmc/host/atmel-mci.c as it is only used in one file

Signed-off-by: Balamanikandan Gunasundar <balamanikandan.gunasundar@microchip.com>
Reviewed-by: Ludovic Desroches <ludovic.desroches@microchip.com>
Link: https://lore.kernel.org/r/20230825095157.76073-3-balamanikandan.gunasundar@microchip.com
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>