include: sbi: Add trap_context pointer in struct sbi_scratch

To track nested traps, the struct sbi_scratch needs a pointer the
current trap context so add trap_context pointer in struct sbi_context.

Signed-off-by: Anup Patel <apatel@ventanamicro.com>
Reviewed-by: Samuel Holland <samuel.holland@sifive.com>
Tested-by: Samuel Holland <samuel.holland@sifive.com>