Added EFR32xG28 device

[shontal1005]

Add hal support for the EFR32xG28 Silicon Labs device

[jhedberg]

@shontal1005 thanks for the PR! However, please follow the same commit message (and PR description) guidelines as for the main tree. Also, please follow the steps documented here to reference this PR from your main tree PR: https://docs.zephyrproject.org/latest/develop/modules.html#process-for-submitting-changes-to-existing-modules

[asmellby]

I'm wondering if it would be better to build upon PR #95 for this, it already adds FG28 and does all the common things for xG28, so extending to ZG28 would be an incremental change. The reason it's not merged yet is that there is no corresponding main tree PR to demonstrate that it doesn't break existing devices.

[janchri]

fyi: I am bit struggeling on enabling the UART shell on the PG23 devKit. For PG28 DevKit everything seems to work fine.