include: Add OPENSBI_EXTERNAL_SBI_TYPES in sbi_types.h

Add OPENSBI_EXTERNAL_SBI_TYPES macro to allow external definitions of data
types and common macros. Also move some common definitions from sbi_bits.h to sbi_types.h.

Signed-off-by: Abner Chang <abner.chang@hpe.com>
Cc: Leif Lindholm <leif.lindholm@linaro.org>
Reviewed-by: Anup Patel <anup.patel@wdc.com>