lib: sbi_ecall: Generate extensions list with carray

Instead of hard-coding the list of extensions in C code, use carray to
generate the list of extensions.

Using carray makes adding and removing extensions slightly cleaner. This
also paves the way for using Kconfig to disable unneeded extensions.

Signed-off-by: Vivian Wang <dramforever@live.com>
Reviewed-by: Andrew Jones <ajones@ventanamicro.com>
Reviewed-by: Anup Patel <anup@brainfault.org>