U5SeriesPkg: Switch to generic OpenSBI platform

New platform files require more space in SEC.
Behavior is determined not by source code but by device tree.

Cc: Daniel Schaefer <daniel.schaefer@hpe.com>
Cc: Abner Chang <abner.chang@hpe.com>
Cc: Sunil V L <sunilvl@ventanamicro.com>
Reviewed-by: Abner Chang <abner.chang@hpe.com>

Signed-off-by: Daniel Schaefer <daniel.schaefer@hpe.com>