To simplify processing, the existed SBI function code is just
invoking the ecall and return the error value. This omits
potential return value in "a1", which is defined in SBI doc.
Add the "sbiret" struct so it can handle all the return value.
In addition, there are some minor improvement:
1. move the SBI related interface to a separate file to make it clean.
2. add all necessary SBI ecall interface macro until version 2.0.
3. add an utilty function to convert sbi error numner to standard
error number.
Signed-off-by: Inochi Amaoto <inochiama@outlook.com>
caeb1757f6