drm: v3d: correct reference to config ARCH_BRCMSTB

Commit 57692c94 ("drm/v3d: Introduce a new DRM driver for Broadcom V3D
V3.x+") adds the config DRM_V3D, which depends on "ARCH_BCMSTB".

Although, a bit confusing: all Broadcom architectures in
./arch/arm/mach-bcm/Kconfig have the prefix "ARCH_BCM", except for
ARCH_BRCMSTB, i.e., the config for Broadcom BCM7XXX based boards.

So, correct the reference ARCH_BCMSTB to the intended ARCH_BRCMSTB.

Signed-off-by: Lukas Bulwahn <lukas.bulwahn@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20210819112253.16484-4-lukas.bulwahn@gmail.com