include: Move RISCV_TRAP_REGS_xyz defines to sbi_trap.h

The struct sbi_trap_regs related defines RISCV_TRAP_REGS_xyz should
be in sbi_trap.h so that we can keep these defines in-sync with changes
in struct sbi_trap_regs.

Signed-off-by: Anup Patel <anup.patel@wdc.com>