lib: Add dynamic registration of SBI extensions

This patch extends our SBI ecall implementation to allow
dynamic registration of various SBI extensions. Using this
dynamic registration we can break-up SBI ecall implementation
into multiple files and even register experimental/custom
SBI extensions from platform code.

Signed-off-by: Anup Patel <anup.patel@wdc.com>
Reviewed-by: Atish Patra <atish.patra@wdc.com>