fsp: update inline assembly codes of ContinuationFunc

U-Boot on x86 is using EAX/EDX/ECX to pass the first 3 parameters to
functions but FSP is using stack to pass parameters. The inline
assembly codes of ContinuationFunc were originally using stack and
now has been updated to use the U-Boot calling convention.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>