Fix modpost Section mismatch error in next_platform_timer().

  [...]
  WARNING: modpost: vmlinux.o(.text.unlikely+0x26e60): Section mismatch in reference from the function next_platform_timer() to the variable .init.data:acpi_gtdt_desc
  The function next_platform_timer() references
  the variable __initdata acpi_gtdt_desc.
  This is often because next_platform_timer lacks a __initdata
  annotation or the annotation of acpi_gtdt_desc is wrong.

  WARNING: modpost: vmlinux.o(.text.unlikely+0x26e64): Section mismatch in reference from the function next_platform_timer() to the variable .init.data:acpi_gtdt_desc
  The function next_platform_timer() references
  the variable __initdata acpi_gtdt_desc.
  This is often because next_platform_timer lacks a __initdata
  annotation or the annotation of acpi_gtdt_desc is wrong.

  ERROR: modpost: Section mismatches detected.
  Set CONFIG_SECTION_MISMATCH_WARN_ONLY=y to allow them.
  make[1]: *** [scripts/Makefile.modpost:59: vmlinux.symvers] Error 1
  make[1]: *** Deleting file 'vmlinux.symvers'
  make: *** [Makefile:1176: vmlinux] Error 2
  [...]

Fixes: a712c3ed9b8a ("acpi/arm64: Add memory-mapped timer support in GTDT driver")
Signed-off-by: Jackie Liu <liuyun01@kylinos.cn>
Acked-by: Hanjun Guo <guohanjun@huawei.com>
Link: https://lore.kernel.org/r/20210823092526.2407526-1-liu.yun@linux.dev
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>