lib: sbi: Convert hart features into hart extensions

Since past few years, we have been using "hart features" in OpenSBI
to represent all optionalities and multi-letter extensions defined
by the RISC-V specifications.

The RISC-V profiles specification has taken a different approach and
started assigning extension names for all optionalities which did not
have any extension name previously.
(Refer, https://github.com/riscv/riscv-profiles/blob/main/profiles.adoc

)

Inspired from the RISC-V profiles specification, we convert OpenSBI
hart features into hart extensions. Going forward, we align the
extension naming with RISC-V profiles specification. Currently, only
"time CSR" and "AIA CSR" have not been assigned extension name but
for everything else we have a name.

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