docs: kgdb.rst: fix :c:type: usages

Which Sphinx 3, :c:type:  can't be used anymore for structs,
as this should be used only for typedefs.

Rely on automarkup.py for struct references.

This file has an special case, though: it uses the tag also
to point to an array. Let's use, instead, :c:expr: for such
purpose, as it should do the right thing.

This should fix this warning:

	./Documentation/dev-tools/kgdb.rst:875: WARNING: Unparseable C cross-reference: 'kdb_poll_funcs[]'
	Invalid C declaration: Expected end of definition. [error at 14]
	  kdb_poll_funcs[]
	  --------------^
Acked-by: Daniel Thompson <daniel.thompson@linaro.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>

Documentation/dev-tools/kgdb.rst

@@ -726,7 +726,7 @@ The kernel debugger is organized into a number of components:
    -  contains an arch-specific trap catcher which invokes
       kgdb_handle_exception() to start kgdb about doing its work
 
-   -  translation to and from gdb specific packet format to :c:type:`pt_regs`
+   -  translation to and from gdb specific packet format to struct pt_regs
 
    -  Registration and unregistration of architecture specific trap
       hooks
@@ -846,7 +846,7 @@ invokes a callback in the serial core which in turn uses the callback in
 the UART driver.
 
 When using kgdboc with a UART, the UART driver must implement two
-callbacks in the :c:type:`struct uart_ops <uart_ops>`.
+callbacks in the struct uart_ops.
 Example from ``drivers/8250.c``::
 
 
@@ -875,7 +875,7 @@ kernel when ``CONFIG_KDB_KEYBOARD=y`` is set in the kernel configuration.
 The core polled keyboard driver for PS/2 type keyboards is in
 ``drivers/char/kdb_keyboard.c``. This driver is hooked into the debug core
 when kgdboc populates the callback in the array called
-:c:type:`kdb_poll_funcs[]`. The kdb_get_kbd_char() is the top-level
+:c:expr:`kdb_poll_funcs[]`. The kdb_get_kbd_char() is the top-level
 function which polls hardware for single character input.
 
 kgdboc and kms