riscv: zicfiss / zicfilp enumeration

riscv: zicfiss / zicfilp enumeration

This patch adds support for detecting the RISC-V ISA extensions
Zicfiss and Zicfilp.  Zicfiss and Zicfilp stand for the unprivileged
integer spec extensions for shadow stack and indirect branch tracking,
respectively.

This patch looks for Zicfiss and Zicfilp in the device tree and
accordingly lights up the corresponding bits in the cpu feature
bitmap. Furthermore this patch adds detection utility functions to
return whether shadow stack or landing pads are supported by the cpu.

Reviewed-by: Zong Li <zong.li@sifive.com>
Reviewed-by: Alexandre Ghiti <alexghiti@rivosinc.com>
Signed-off-by: Deepak Gupta <debug@rivosinc.com>
Tested-by: Andreas Korb <andreas.korb@aisec.fraunhofer.de> # QEMU, custom CVA6
Tested-by: Valentin Haudiquet <valentin.haudiquet@canonical.com>
Link: https://patch.msgid.link/20251112-v5_user_cfi_series-v23-3-b55691eacf4f@rivosinc.com
[pjw@kernel.org: updated to apply; cleaned up patch description]
Signed-off-by: Paul Walmsley <pjw@kernel.org>