LoongArch: Fix section mismatch due to acpi_os_ioremap()

Now acpi_os_ioremap() is marked with __init because it calls memblock_
is_memory() which is also marked with __init in the !ARCH_KEEP_MEMBLOCK
case. However, acpi_os_ioremap() is called by ordinary functions such
as acpi_os_{read, write}_memory() and causes section mismatch warnings:

WARNING: modpost: vmlinux.o: section mismatch in reference: acpi_os_read_memory (section: .text) -> acpi_os_ioremap (section: .init.text)
WARNING: modpost: vmlinux.o: section mismatch in reference: acpi_os_write_memory (section: .text) -> acpi_os_ioremap (section: .init.text)

Fix these warnings by selecting ARCH_KEEP_MEMBLOCK unconditionally and
removing the __init modifier of acpi_os_ioremap(). This can also give a
chance to track "memory" and "reserved" memblocks after early boot.

Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>