RISC-V: KVM: Move copy_sbi_ext_reg_indices() to SBI implementation

RISC-V: KVM: Move copy_sbi_ext_reg_indices() to SBI implementation

The ONE_REG handling of SBI extension enable/disable registers and
SBI extension state registers is already under SBI implementation.
On similar lines, let's move copy_sbi_ext_reg_indices() under SBI
implementation.

Signed-off-by: Anup Patel <apatel@ventanamicro.com>
Reviewed-by: Andrew Jones <ajones@ventanamicro.com>
Link: https://lore.kernel.org/r/20250823155947.1354229-5-apatel@ventanamicro.com
Signed-off-by: Anup Patel <anup@brainfault.org>