include: Add separate header for OpenSBI version

Currently, the OpenSBI version is in top-level Makefile so
firmware linking to OpenSBI static library have no-way to
know OpenSBI version.

This patch moves OpenSBI version from top-level Makefile to
sbi/sbi_version.h header which provides OPENSBI_VERSION_MAJOR
and OPENSBI_VERSION_MINOR defines.

NOTE: the SBI spec (or SBI ecall interface) version is
different. The SBI spec version is provided by functions
sbi_ecall_version_major() and sbi_ecall_version_minor().

Signed-off-by: Anup Patel <anup.patel@wdc.com>