Platform/SiFive/U5SeriesPkg: Remove use of deprecated interfaces

The non-status reporting PcdSet functions were deprecated and have
now been removed. Update SiFive code to assert on error status.

Deprecated PcdSet* code under DISABLE_NEW_DEPRECATED_INTERFACES has
been removed. This includes PcdSetXX interfaces used by RISC-V code
which is now causing a compilation error. Switch to the PcdSetXXS
interfaces.

Cc: Leif Lindholm <leif@nuviainc.com>
Cc: Abner Chang <abner.chang@hpe.com>
Cc: Daniel Schaefer <daniel.schaefer@hpe.com>
Cc: Gilbert Chen <gilbert.chen@hpe.com>
Signed-off-by: Mark Salter <msalter@redhat.com>
Reviewed-by: Abner Chang <abner.chang@hpe.com>