regmap: add missing prototype for devm_init_slimbus

For some reason the devm variant of slimbus init is not added
into the header eventhough this __devm_regmap_init_slimbus()
is an exported function.

This patch adds this. This also fixes below warning in regmap-slimbus.c
regmap-slimbus.c:65:15: warning: symbol '__devm_regmap_init_slimbus'
 was not declared. Should it be static?
regmap-slimbus.c:65:16: warning: no previous prototype for
 '__devm_regmap_init_slimbus' [-Wmissing-prototypes]

Fixes: 7d6f7fb0 ("regmap: add SLIMbus support")
Signed-off-by: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>
Signed-off-by: Mark Brown <broonie@kernel.org>