MFC r360008-r360009, r360011

r360008:
mmc_fdt_helpers: Always init the timout

We use the taskqueue to schedule card detection so always init it.
This is a proper solution instead of r359965.

MFH: r359924

r360009:
mmc_fdt_helpers: Drain the cd pin taskqueue in mmc_fdt_gpio_teardown

We have no use for it now.

r360011:
arm: allwinner: aw_mmc: Make it possible to unload the module

While here, add a makefile in sys/modules/allwinner so it is built.
Also add the PNP info so devmatch will load this module automatically.