lib: utils/regmap: Mark syscon region as shared read-write

The syscon region used by OpenSBI should be marked as a shared
read-write region between M-mode and SU-mode.

Signed-off-by: Anup Patel <apatel@ventanamicro.com>
Signed-off-by: Mayuresh Chitale <mchitale@ventanamicro.com>