EFI: Update the documentation to reflect the correct value of OsIndications

The OsIndications is a 64 bit variable, and the current code expects
the value of the variable to be 64 bit. Update the documentation to
reflect this fact.

Signed-off-by: Sughosh Ganu <sughosh.ganu@linaro.org>
Reviewed-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>