metal_dcache_l1_flush() should not issue fence.i by default

[TacoGrandeTX]

Our metal_dcache_l1_flush() contains a fence.i instruction that is only needed if one is making code side updates through the data cache - namely pushing out new instructions to memory:

https://github.com/sifive/freedom-metal/blob/v201908-branch/src/cache.c#L135

The fence.i invalidates the I Cache and can negatively impact performance. If a user is pushing out a code update they should call fence.i on their own or there should be a new argument added which specifies if the fence.i should be executed.

Customers don't appreciate having the I Cache invalidated when not necessary.