mmc: tmio: rename tmio_mmc_{pio => core}.c

Rename tmio_mmc_pio.c to tmio_mmc_core.c to more accurately reflect its
function: to provide core code for the tmio-mmc and sh-mobole-sdhi drivers.

Signed-off-by: Simon Horman <horms+renesas@verge.net.au>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>