mfd: syscon: Fix syscon_regmap_lookup_by_phandle_args() dummy

If CONFIG_MFD_SYSCON=n:

    include/linux/mfd/syscon.h:54:23: warning: ‘syscon_regmap_lookup_by_phandle_args’ defined but not used [-Wunused-function]

Fix this by adding the missing inline keyword.

Fixes: 6a24f567 ("mfd: syscon: Add arguments support for syscon reference")
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Lee Jones <lee.jones@linaro.org>