ArmPlatformPkg/Bds: Use HandleProtocol to get SNP instance

ArmPlatformPkg/Bds: Use HandleProtocol to get SNP instance

LocateProtocol only gets the 1st SNP instance and this will be wrong
in a system with multiple SNP instances installed. Use HandleProtocol
instead.

Cc: Olivier Martin <olivier.martin@arm.com>
Cc: Leif Lindholm <leif.lindholm@linaro.org>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Heyi Guo <heyi.guo@linaro.org>
Reviewed-by: Olivier Martin <Olivier.Martin@arm.com>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@18030 6f19259b-4bc3-4df7-8a09-765794883524