Convert CONFIG_SYS_MMC_MAX_DEVICE to Kconfig

This converts the following to Kconfig:
   CONFIG_SYS_MMC_MAX_DEVICE

Signed-off-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Jaehoon Chung <jh80.chung@samsung.com>