arch/arm: add options for FPv5 FPU

arch/arm: add options for FPv5 FPU

Contrary to its older brother, the FPv5 comes in two flavours; single-
and double-precision [0] [1]. the two variants are only available for
cortex-m7 cores, and the two variants are known to gcc as fpv5-sp-d16
and fpv5-d16, respectively, since gcc-5 [2].

[0] https://en.wikipedia.org/wiki/ARM_Cortex-M#Cortex-M7
[1] https://developer.arm.com/docs/ddi0489/latest/floating-point-unit
[2] https://gcc.gnu.org/git/?p=gcc.git;a=commit;h=a076f99fa702deac764f6e0441b9435ad999f521

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