Reserved values for ARCACHE and ARBURST

This assertion checks all AR transactions to ensure that reserved attributes are not used. This assertion will check the following rule:

• The start of read transaction implies that ARBURST!=2'b11, and
• ARCACHE is not an invalid combination (4'b010x, 4'b100x, and 4'b110x).

These could be split into two separate assertions or combined into a single assertion. For the tutorial, we combine them to show how to build more complex assertions.

This assertion is triggered when ARVALID==1 and ARREADY==1 (acceptance of a read address).