arch/arm: introduce generic FPU internal option

arch/arm: introduce generic FPU internal option

Currently, we consider that any VFP FPU is a superset of VFPv2, and thus
we use VFPv2 as a way to detect that a VFP is used.

However, for Cortex-M cores, the optional FPU is not a superset of
VFPv2; it is even not a VFP [0].

As a consequence, we can no longer consider VFPv2 as a indication that
an FPU is present.

So, we introduce two new internal options, BR2_ARM_CPU_MAYBE_HAS_FPU and
BR2_ARM_CPU_HAS_FPU, which we use to consider the presence of an FPU.

[0] https://en.wikipedia.org/wiki/ARM_Cortex-M#Cortex-M4

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>