kmod-regmap: make regmap-mmio user selectable

REGMAP_MMIO is a hidden symbol, so it cannot be directly selected. If nothing
selects it, it will fail the build with:

ERROR: module '/store/buildbot/slave/slave/rb532/build/build_dir/target-mipsel_mips32_musl-1.1.10/linux-rb532/linux-3.18.19/drivers/base/regmap/regmap-mmio.ko' is missing.
make[2]: *** [/store/buildbot/slave/slave/rb532/build/bin/rb532/packages/base/kmod-regmap_3.18.19-1_rb532.ipk] Error 1

Fix this by adding a description to allow selecting it manually.

Fixes: r46451 ("kmod-regmap: enable MMIO support")
Signed-off-by: Jonas Gorski <jogo@openwrt.org>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@46485 3c298f89-4303-0410-b956-a3cf2f4a3e73