cargo: bump safe-mmio in /src/exercises/bare-metal/rtc

Bumps [safe-mmio](https://github.com/google/safe-mmio) from 0.2.7 to 0.3.0.
- [Release notes](https://github.com/google/safe-mmio/releases)
- [Changelog](https://github.com/google/safe-mmio/blob/main/CHANGELOG.md)
- [Commits](google/safe-mmio@0.2.7...0.3.0)

---
updated-dependencies:
- dependency-name: safe-mmio
  dependency-version: 0.3.0
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]

src/exercises/bare-metal/rtc/Cargo.lock

@@ -14,7 +14,7 @@ arm-pl011-uart = "0.4.0"
 bitflags = "2.11.0"
 chrono = { version = "0.4.44", default-features = false }
 log = "0.4.29"
-safe-mmio = "0.2.7"
+safe-mmio = "0.3.0"
 smccc = "0.2.2"
 spin = "0.10.0"
 zerocopy = "0.8.48"