target/riscv: Preliminary textra trigger CSR writting support

target/riscv: Preliminary textra trigger CSR writting support

This commit allows program to write textra trigger CSR for type 2, 3, 6
triggers. In this preliminary patch, the textra.MHVALUE and the
textra.MHSELECT fields are allowed to be configured. Other fields, such
as textra.SBYTEMASK, textra.SVALUE, and textra.SSELECT, are hardwired to
zero for now.

For textra.MHSELECT field, the only legal values are 0 (ignore) and 4
(mcontext). Writing 1~3 into textra.MHSELECT will be changed to 0, and
writing 5~7 into textra.MHSELECT will be changed to 4. This behavior is
aligned to RISC-V SPIKE simulator.

Signed-off-by: Alvin Chang <alvinga@andestech.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-ID: <20240826024657.262553-2-alvinga@andestech.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>