arm: add SOC_BRCM_BCM2837 option, include it in GENERIC

We use armv7/GENERIC for the RPI2 images. The original RPI2 is actually a
32-bit BCM2836, but v1.2 was upgraded to the 64-bit BCM2837. The project
continues to provide the RPI2 image as armv7, as it's the lowest common
denominator of the two. Historically, we've just kind of implicitly
acknowledged this by including some bcm2837 bits on a SOC_BCM2836 kernel
config -- this worked until r354875 added code that actually cared.

Acknowledge formally that BCM2837 is valid in arm32.

This name is inconsistent with the other BCM* SOC on !arm64 for two reasons:

1. It's a pre-existing option on arm64, and
2. the naming convention on arm/ should've arguably changed to include BRCM

#1 seems to be a convincing enough argument to maintain the existing name
for it.