lib: utils/regmap: Add generic regmap access library

We add generic regmap access library which is independent of
hardware description format (FDT or ACPI). The OpenSBI platform
support or regmap drivers can register regmap instances which
can be discovered by different regmap clients based on the
unique ID of regmap instances.

Signed-off-by: Anup Patel <apatel@ventanamicro.com>
Reviewed-by: Xiang W <wxjstz@126.com>