drm/amd/include/vega10_ip_offset: Mark _BASE structs as __maybe_unused

This patch fixes nearly 400 warnings!

These structures are too widely used in too many varying
configurations to be split-up into different headers or moved into
source files.

Instead, we'll mark them as __maybe_unused which tells the compiler
that we're aware they're being included into source files which do not
make use of them - but we've looked into it, and it's okay.

Let's tidy-up whilst were here.  Just alignment stuff.

Fixes the following W=1 kernel build warning(s):

 In file included from drivers/gpu/drm/amd/amdgpu/navi14_reg_init.c:27:
 drivers/gpu/drm/amd/amdgpu/../include/navi14_ip_offset.h:179:29: warning: ‘USB0_BASE’ defined but not used [-Wunused-const-variable=]
 179 | static const struct IP_BASE USB0_BASE ={ { { { 0x0242A800, 0x05B00000, 0, 0, 0 } },
 | ^~~~~~~~~
 drivers/gpu/drm/amd/amdgpu/../include/navi14_ip_offset.h:172:29: warning: ‘UMC_BASE’ defined but not used [-Wunused-const-variable=]
 172 | static const struct IP_BASE UMC_BASE ={ { { { 0x00014000, 0x02425800, 0, 0, 0 } },
 | ^~~~~~~~
 drivers/gpu/drm/amd/amdgpu/../include/navi14_ip_offset.h:151:29: warning: ‘SDMA_BASE’ defined but not used [-Wunused-const-variable=]
 151 | static const struct IP_BASE SDMA_BASE ={ { { { 0x00001260, 0x0000A000, 0x02402C00, 0, 0 } },
 | ^~~~~~~~~
 drivers/gpu/drm/amd/amdgpu/../include/navi14_ip_offset.h:144:29: warning: ‘PCIE0_BASE’ defined but not used [-Wunused-const-variable=]
 144 | static const struct IP_BASE PCIE0_BASE ={ { { { 0x00000000, 0x00000014, 0x00000D20, 0x00010400, 0x0241B000 } },
 | ^~~~~~~~~~

NB: Snipped for brevity

Cc: Alex Deucher <alexander.deucher@amd.com>
Cc: "Christian König" <christian.koenig@amd.com>
Cc: David Airlie <airlied@linux.ie>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: amd-gfx@lists.freedesktop.org
Cc: dri-devel@lists.freedesktop.org
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>