[entropy_src,sw] Make KAT test more robust
[entropy_src,sw] Make KAT test more robust

The entropy_src KAT test's use of non-blocking data reads is important.
However it leaves the test vulnerable to false failures due to timing
delays within the IP.   This commit issues a number of retry attempts
whenever the ENTROPY_DATA register is read.

Signed-off-by: Martin Lueker-Boden <7e82c17fafb3b7b91b3c3396a2f21a060e5a6c3d@wdc.com>

lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan