powerpc/rtas: export rtas_error_rc() for reuse.

Also, #define descriptive names for common rtas return codes and use it
instead of numeric values.

Signed-off-by: Mahesh Salgaonkar <mahesh@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/169235811556.193557.1023625262204809514.stgit@jupiter