OMAP's hs_mmc driver is used for MMC controller operation on many
omap2plus SoCs (OMAP2430, OMAP3, 4, 5 and AM335x).

Considering that the device tree entries are already present for these,
allow the driver to be built using the config ARCH_OMAP2PLUS rather
than individually adding a config for each SoC to enable the
support.

Use COMPILE_TEST to enable the build for other platforms.
Signed-off-by: Amarinder Bindra <a-bindra@ti.com>
Cc: Ezequiel Garcia <ezequiel.garcia@free-electrons.com>
Cc: Nishanth Menon <nm@ti.com>
Acked-by: Felipe Balbi <balbi@ti.com>
Acked-by: Balaji T K <balajitk@ti.com>
Signed-off-by: Chris Ball <cjb@laptop.org>