arch: arm: aarch32: Remove CPU type dependency from CONFIG_FP16

The commit 434ca63e introduced the
Cortex-A and Cortex-R CPU type dependency to `CONFIG_FP16` based on
the reasoning that the hardware half-precision support is only
available on them.

While it is true that the _hardware_ half-precision support is limited
to these targets, the compiler will provide the _software_ emulation
for the targets that lack the hardware half-precision support, as
mentioned in 41fd6e00 (the original
commit that introduced `CONFIG_FP16`).

Signed-off-by: Stephanos Ioannidis <stephanos.ioannidis@nordicsemi.no>