MAINTAINERS: Update Manivannan Sadhasivam email address

My Linaro email is going to bounce soon, so switch to the kernel.org alias
and add relevant .mailmap entry.

[bhelgaas: squash https://patch.msgid.link/20250604120833.32791-3-manivannan.sadhasivam@linaro.org

]
Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Neil Armstrong <neil.armstrong@linaro.org>
Link: https://patch.msgid.link/20250604120833.32791-2-manivannan.sadhasivam@linaro.org