ACPICA: Introduce acpi_hw_gpe_read() and acpi_hw_gpe_write()

Now that GPE blocks are validated at the initialization time, accesses
to GPE registers can be made more straightforward by ommitting all of
the redundant checks in acpi_hw_read() and acpi_hw_write() and only
invoking the OS-provided helper for the given type of access (read or
write) and the address space holding these registers.

For this reason, introduce simplified routines for accessing GPE
registers, acpi_hw_gpe_read() and acpi_hw_gpe_write(), designed in
accordance with the above observation, and modify all of the code
accessing GPE registers to use them instead of acpi_hw_read() and
acpi_hw_write(), respectively.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>