lib: utils/sys: Extend HTIF library to allow custom base address

Some of RISC-V emulators provide HTIF at fixed base address so for
such emulators users have to hard-code HTIF base address in the
linker script.

To address this problem, we let users optionally provide fixed HTIF
base address via platform support (or device tree).

Signed-off-by: Anup Patel <apatel@ventanamicro.com>
Reviewed-by: Dong Du <Dd_nirvana@sjtu.edu.cn>
Reviewed-by: Atish Patra <atishp@rivosinc.com>