riscv: cpufeature: Use pre-defined ISA ext macros to index isa2hwcap

riscv: cpufeature: Use pre-defined ISA ext macros to index isa2hwcap

We have pre-defined ISA extension macros, here use those macros to
replace a magic number for isa2hwcap definition and some array
indexing for isa2hwcap access.

This doesn't change the original functionality, just improve the code
maintainability and readability.

Signed-off-by: Hui Wang <hui.wang@canonical.com>
Link: https://patch.msgid.link/20260506132152.53239-1-hui.wang@canonical.com
Signed-off-by: Paul Walmsley <pjw@kernel.org>