Increase STM32 default deep sleep latency to 4ms

[LMESTM]

Description (required)

Recent measurements have shown that the total wake-up latency can be more
than the 3ms initially set. It was measured as 3,7ms on DISCO_L475VG_IOT01A
target and 3,1ms on NUCLEO_L073RZ target. So set it to 4ms to be on the
safe side.

Summary of change (What the change is for and why)

The change better reflects the reality of the actual latency and will allows mbed-os OS to schedule
more precisely the wake-up timings.

Documentation (Details of any document updates required)

No specific documentation update needed.

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Pull request type (required)

[x] Patch update (Bug fix / Target update / Docs update / Test update / Refactor)
[] Feature update (New feature / Functionality change / New API)
[] Major update (Breaking change E.g. Return code change / API behaviour change)

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Test results (required)

[] No Tests required for this change (E.g docs only update)
[x] Covered by existing mbed-os tests (Greentea or Unittest)
[] Tests / results supplied as part of this PR

Note that this patch actually will fix the initial issue raised in #11545

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Reviewers (optional)

@jeromecoutant @ABOSTM @TuomoHautamaki @kjbracey-arm

[0xc0170]

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