gpio: syscon: reduce message level when direction reg offset not in dt

Now GPIO syscon driver produces bunch of warnings during the
boot of Kesytone 2 SoCs:
 gpio-syscon soc:keystone_dsp_gpio@02620240: can't read the dir register offset!
 gpio-syscon soc:keystone_dsp_gpio@2620244: can't read the dir register offset!

This message unintentionally was added using dev_err(), but its
actual log level is debug, because third cell of "ti,syscon-dev" is
optional.

Hence change it to dev_dbg() as it should be.

This patch fixes commit:
 5a3e3f88 ("gpio: syscon: retriave syscon node and regs offsets from dt")

Reported-by: Russell King <linux@arm.linux.org.uk>
Tested-by: Murali Karicheri <m-karicheri2@ti.com>
Acked-by: Santosh Shilimkar <ssantosh@kernel.org>
Signed-off-by: Grygorii Strashko <grygorii.strashko@linaro.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>