mmc: sdhci: Split sdhci_add_host()

Split sdhci-add_host() in order to further our objective to make
sdhci into a library.

The split divides code that sets up mmc and sdhci parameters, from
code that actually activates things - such as tasklet initialization,
requesting the irq, and adding (and starting) the host.

This gives drivers an opportunity to change various settings before
committing to start the host.

Drivers can continue to call sdhci_add_host() but drivers that want
to take advantage of the split instead call sdhci_setup_host() followed
by __sdhci_add_host().

Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>