lib: sbi: Detect hart features only once for each hart

Currently, the hart_detect_features() is called everytime a hart
is stopped and started again which is unnecessary work.

We update hart_detect_features() to detect hart features only
once for each hart.

Signed-off-by: Anup Patel <apatel@ventanamicro.com>
Reviewed-by: Atish Patra <atishp@rivosinc.com>