✨ add optional XIP cache

[stnolting]

As discussed in #793 this PR adds an optional and configurable cache to the XIP module. A new set of top generics if provided to configure it:

 -- Execute in-place module (XIP) --
 XIP_EN               : boolean                  := false; -- implement execute in place module (XIP)?
 XIP_CACHE_EN         : boolean                  := false; -- implement XIP cache?
 XIP_CACHE_NUM_BLOCKS : natural range 1 to 256   := 8;     -- number of blocks (min 1), has to be a power of 2
 XIP_CACHE_BLOCK_SIZE : natural range 1 to 2**16 := 256;   -- block size in bytes (min 4), has to be a power of 2

The cache is implemented as simple direct-mapped read-only cache.