refactor(lib/smc): Use #define for RSI error codes

The error code enums are changed to #defines so that the
same header can be used in C and assembler. The enum
rsi_status_t was never used hence the type is deleted.

Signed-off-by: Yousuf A <yousuf.sait@arm.com>
Signed-off-by: Soby Mathew <soby.mathew@arm.com>
Change-Id: Ic0c21620b20c5a20c277e401e80969c73aeff1d1