lib: sbi: Allow ecall handlers to directly update register state

Some of the upcoming SBI extensions (such as SSE) will directly
update register state so improve the prototype of ecall handler
to accommodate this. Further, this flexibility allows us to
push the trap redirection from sbi_ecall_handler() to the
sbi_ecall_legacy_handler().

Signed-off-by: Anup Patel <apatel@ventanamicro.com>