ArmPlatformPkg/SP804TimerLib: Fix delay functions to be reentrant

ArmPlatformPkg/SP804TimerLib: Fix delay functions to be reentrant

In the previous implementation, if the MicroSecondDelay or NanoSecondDelay
were called while a delay function was already running, the timer HW controller
was reinitialized with the new settings and it was discarding the settings
of the first called delay function.

git-svn-id: https://edk2.svn.sourceforge.net/svnroot/edk2/trunk/edk2@12163 6f19259b-4bc3-4df7-8a09-765794883524