sim: Fix Temperature class
sim: Fix Temperature class

* Adding __str__ method: To fix its printing on config.ini
(Replacing <m5.params.Temperature> with the Temperature value)

* Replacing "fromKelvin" with from_kelvin
(that's how pybind exports it)

* Fixing config_value to allow JSON serialization
(JIRA: https://gem5.atlassian.net/browse/GEM5-951)

Change-Id: I1aaea9c9df6466a5cbed0a29c5937243796948d2
Signed-off-by: Giacomo Travaglini <8eef13a4f970e2e9372c8d26ad0737b760f99c02@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/44167
Reviewed-by: Andreas Sandberg <c9f5686ee5bd561c93f9de79681eee75bfa7f36e@arm.com>
Reviewed-by: Jason Lowe-Power <92de577f37050ad05b60626d2f17913f40dcb1a2@gmail.com>
Maintainer: Andreas Sandberg <c9f5686ee5bd561c93f9de79681eee75bfa7f36e@arm.com>
Tested-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
(cherry picked from commit 6cb9c3e87fa8034122310613079ae4f058f93233)
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/44745
Maintainer: Jason Lowe-Power <92de577f37050ad05b60626d2f17913f40dcb1a2@gmail.com>

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