riscv: Code optimization for SHA-256.

Keep SHA-256 constant values in registers to save the loading time.

Move the constant loading for sha256 into a separate subroutine.
By creating a dedicated sub routine for loading sha256 constants, the
code can be made more modular and easier to modify in the future.

Relaxing the SHA256 constraint, zvknhb also supports SHA256.

Simplify the H and mask initialization flows.

Signed-off-by: Phoebe Chen <phoebe.chen@sifive.com>

Reviewed-by: Tomas Mraz <tomas@openssl.org>
Reviewed-by: Paul Dale <pauli@openssl.org>
Reviewed-by: Hugo Landau <hlandau@openssl.org>
(Merged from #21923)