docs: Be consistent about capitalization of 'Arm'

docs: Be consistent about capitalization of 'Arm'

The company 'Arm' went through a rebranding some years back
involving a recapitalization from 'ARM' to 'Arm'. As a result
our documentation is a bit inconsistent between the two forms.
It's not worth trying to update everywhere in QEMU, but it's
easy enough to make docs/ consistent.

Note that "ARMv8" and similar architecture names, and
older CPU names like "ARM926" still retain all-caps.

Signed-off-by: Peter Maydell <[email protected]>
Reviewed-by: Alex Bennée <[email protected]>
Reviewed-by: Niek Linnenbank <[email protected]>
Message-id: 20200309215818[email protected]