mmc: tmio: ioremap memory resource in tmio_mmc_host_alloc()

The register region is ioremap'ed in the tmio_mmc_host_probe(), i.e.
drivers cannot get access to the hardware before mmc_add_host().

Actually, renesas_sdhi_core.c reads out the CTL_VERSION register to
complete the platform-specific settings.  However, at this point,
the MMC host is already running.

Move the register ioremap to tmio_mmc_host_alloc() so that drivers
can perform platform-specific settings between tmio_mmc_host_alloc()
and tmio_mmc_host_probe().

I changed tmio_mmc_host_alloc() to return an error pointer to
propagate the return code from devm_ioremap_resource().

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Reviewed-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>