scripts: kernel-doc: parse next structs/unions

There are several places within the Kernel tree with nested
structs/unions, like this one:

struct ingenic_cgu_clk_info {
const char *name;
enum {
CGU_CLK_NONE = 0,
CGU_CLK_EXT = BIT(0),
CGU_CLK_PLL = BIT(1),
CGU_CLK_GATE = BIT(2),
CGU_CLK_MUX = BIT(3),
CGU_CLK_MUX_GLITCHFREE = BIT(4),
CGU_CLK_DIV = BIT(5),
CGU_CLK_FIXDIV = BIT(6),
CGU_CLK_CUSTOM = BIT(7),
} type;
int parents[4];
union {
struct ingenic_cgu_pll_info pll;
struct {
struct ingenic_cgu_gate_info gate;
struct ingenic_cgu_mux_info mux;
struct ingenic_cgu_div_info div;
struct ingenic_cgu_fixdiv_info fixdiv;
};
struct ingenic_cgu_custom_info custom;
};
};

Currently, such struct is documented as:

**Definition**

::
struct ingenic_cgu_clk_info {
const char * name;
};

**Members**

``name``
name of the clock

With is obvioulsy wrong. It also generates an error:
drivers/clk/ingenic/cgu.h:169: warning: No description found for parameter 'enum'

However, there's nothing wrong with this kernel-doc markup: everything
is documented there.

It makes sense to document all fields there. So, add a
way for the core to parse those structs.

With this patch, all documented fields will properly generate
documentation.

Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>