mfd: syscon: Fix sparse warning

Fix the following sparse warnings:

drivers/mfd/syscon.c:40:15: warning: symbol 'syscon_node_to_regmap' was not declared. Should it be static?
drivers/mfd/syscon.c:56:15: warning: symbol 'syscon_regmap_lookup_by_compatible' was not declared. Should it be static?
drivers/mfd/syscon.c:72:15: warning: symbol 'syscon_regmap_lookup_by_phandle' was not declared. Should it be static?

Cc: Dong Aisheng <dong.aisheng@linaro.org>
Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>