RISC-V: define the elements of the VCSR vector CSR

RISC-V: define the elements of the VCSR vector CSR

The VCSR CSR contains two elements VXRM[2:1] and VXSAT[0].

Define constants for those to access the elements in a readable way.

Acked-by: Guo Ren <guoren@kernel.org>
Reviewed-by: Conor Dooley <conor.dooley@microchip.com>
Signed-off-by: Heiko Stuebner <heiko.stuebner@vrull.eu>
Signed-off-by: Charlie Jenkins <charlie@rivosinc.com>
Tested-by: Yangyu Chen <cyy@cyyself.name>
Link: https://lore.kernel.org/r/20241113-xtheadvector-v11-6-236c22791ef9@rivosinc.com
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>