pinctrl: st: Enhance the controller to manage unavailable registers

This patch adds a new logic inside the st pinctrl to manage
an unsupported scenario: some sysconfig are not available!

This is the case of STiH407 where, although documented, the
following registers from SYSCFG_FLASH have been removed from the SoC.

SYSTEM_CONFIG3040
   Output Enable pad control for all PIO Alternate Functions
and
SYSTEM_ CONFIG3050
   Pull Up pad control for all PIO Alternate Functions

Without managing this condition an imprecise external abort
will be detect.

To do this the patch also reviews the st_parse_syscfgs
and other routines to manipulate the registers only if
actually available.
In any case, for example the st_parse_syscfgs detected
an error condition but no action was made in the
st_pctl_probe_dt.

Acked-by: Giuseppe Cavallaro <peppe.cavallaro@st.com>
Acked-by: Srinivas Kandagatla <srinivas.kandagatla@st.com>
Signed-off-by: Giuseppe Cavallaro <peppe.cavallaro@st.com>
Signed-off-by: Maxime Coquelin <maxime.coquelin@st.com>
Acked-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>