Add partial capability read support to AXI SRAM

[raylau1]

• Add logic to handle partial reads of capabilities, where the tag valid bit will be clamped to 0
• Remove some assumptions in AXI SRAM (e.g. AW address unchanged during W burst)
• Add smoketest for AXI SRAM

[raylau1]

Changes are on branch improve_axi_sram

[thommythomaso]

Addressed in #390.