ARC: move common ops for line/full cache into helpers

INV cmd for dcache provides 2 modes discard or wback-before-discard.
One is default and other needs to be set, if so desired. This is common
for line-op/entire-cache-op. So refactor them out into a helper

Doesn't affect generated code but paves way for any common
micro-optimization.

Signed-off-by: Vineet Gupta <vgupta@synopsys.com>