LoongArch: Remove CONFIG_ACPI_TABLE_UPGRADE in platform_init()

Both acpi_table_upgrade() and acpi_boot_table_init() are defined as
empty functions under !CONFIG_ACPI_TABLE_UPGRADE and !CONFIG_ACPI in
include/linux/acpi.h, there are no implicit declaration errors with
various configs.

  #ifdef CONFIG_ACPI_TABLE_UPGRADE
  void acpi_table_upgrade(void);
  #else
  static inline void acpi_table_upgrade(void) { }
  #endif

  #ifdef	CONFIG_ACPI
  ...
  void acpi_boot_table_init (void);
  ...
  #else	/* !CONFIG_ACPI */
  ...
  static inline void acpi_boot_table_init(void)
  {
  }
  ...
  #endif	/* !CONFIG_ACPI */

As Huacai suggested, CONFIG_ACPI_TABLE_UPGRADE is ugly and not necessary
here, just remove it. At the same time, just keep CONFIG_ACPI to prevent
potential build errors in future, and give a signal to indicate the code
is ACPI-specific. For the same reason, we also put acpi_table_upgrade()
under CONFIG_ACPI.

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