Silicon/Hisilicon: add PlatformBootManagerUnableToBoot() to PlatformBm

edk2 commit 1010873becc5,
("MdeModulePkg/BdsDxe: Call PlatformBootManagerUnableToBoot()") broke the
d0x build, since they use their own PlatformBootManagerLib, which did not
include this function.

Implement a trivial one, that just returns.

Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Leif Lindholm <leif.lindholm@linaro.org>
Reviewed-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>