ARM: Permit any ARMv6K CPU to read the MVFR0 and MVFR1 VFP registers.

This patch replaces the ARM_FEATURE_VFP3 test when reading MVFR registers
with a test for a new feature flag ARM_FEATURE_MVFR, and sets this feature
for all ARMv6K cores (ARM1156 is not a v6K core, yet supports MVFR; qemu
does not support ARM1156 at this time.)

MVFR0 and MVFR1 were introduced in ARM1136JF-S r1p0 (ARMv6K, VFPv2) and are
present in ARM1156T2F-S (non-v6K), ARM1176JZF-S, ARM11MPCore and newer cores.
Reference: ARM DDI 0211H, 0290G, 0301H, 0360E.
http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.ddi0211h/Ffbefjag.html

Without this change, the linux kernel will not boot with VFP support enabled
under ARM1176 system emulation, due to the unconditional use of MVFR1 at the
end of vfp_init() in arch/arm/vfp/vfpmodule.c:

  VFP support v0.3: implemetor 41 architecture 1 part 20 variant b rev 5
  Internal error: Oops - undefined instruction: 0 [#1]

Signed-off-by: Andrew Towers <atowers@gmail.com>
Reviewed-by: Andreas Färber <afaerber@suse.de>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
