[libcpu][aarch64] fix boot failure when dropping from EL2 in entry_point.S

[Xenithya]

拉取/合并请求描述：(PR description)

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为什么提交这份PR (why to submit this PR)

Fix AArch64 cold boot failure (HPFAR_EL2 Stage-2 fault / panic) when booted directly from U-Boot in EL2 mode.

你的解决方案是什么 (what is your solution)

1. Reordered startup sequence: Executed init_cpu_el before accessing any _el1 system registers (e.g. tpidr_el1) in _start.
2. Fixed CurrentEL logic: Re-evaluated CurrentEL after dropping from EL3 to EL2 in .init_cpu_hyp.
3. Cleaned up EL2 hardware context:
   • Disabled EL2 MMU and Caches (sctlr_el2).
   • Cleared FP/SIMD traps (cptr_el2).
   • Invalidated stale EL2/Stage-2 TLBs (tlbi alle2is, tlbi vmalle1is) to resolve hardware fault traps.

如何测试 / How to test?

• Hardware Platform: AArch64 board (Cortex-A CPU)
• Bootloader: U-Boot (running in EL2 mode, without prior ATF cleanup)
• Test Result: Cold boot successfully enters the msh prompt every time without hanging or throwing HPFAR_EL2 exceptions.

请提供验证的bsp和config (provide the config and bsp)

• BSP:

• .config:

• action:

]

当前拉取/合并请求的状态 Intent for your PR

必须选择一项 Choose one (Mandatory):

• 本拉取/合并请求是一个草稿版本 This PR is for a code-review and is intended to get feedback
• 本拉取/合并请求是一个成熟版本 This PR is mature, and ready to be integrated into the repo

代码质量 Code Quality：

我在这个拉取/合并请求中已经考虑了 As part of this pull request, I've considered the following:

• 已经仔细查看过代码改动的对比 Already check the difference between PR and old code
• 代码风格正确，包括缩进空格，命名及其他风格 Style guide is adhered to, including spacing, naming and other styles
• 没有垃圾代码，代码尽量精简，不包含#if 0代码，不包含已经被注释了的代码 All redundant code is removed and cleaned up
• 所有变更均有原因及合理的，并且不会影响到其他软件组件代码或BSP All modifications are justified and not affect other components or BSP
• 对难懂代码均提供对应的注释 I've commented appropriately where code is tricky
• 代码是高质量的 Code in this PR is of high quality
• 已经使用clang-format 源码格式化工具确保格式符合RT-Thread代码规范 This PR has been formatted with clang-format and complies with RT-Thread code specification
• 如果是新增bsp, 已经添加ci检查到.github/ALL_BSP_COMPILE.json 详细请参考链接BSP自查

[github-actions]

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