x86: apci: Change PM1_CNT register access to RMW

In enter_acpi_mode() PM1_CNT register is changed to PM1_CNT_SCI_EN
directly without preserving its previous value. Update to change
the register access to read-modify-write (RMW).

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Tested-by: Stefan Roese <sr@denx.de>