boards: arm: kconfig: Do not assign promptless CPU_HAS_FPU

Assignments have no effect on promptless symbols. Flagged by
https://github.com/zephyrproject-rtos/zephyr/pull/20742.

CPU_HAS_FPU is enabled through being selected by other symbols.

Also enable floating point support for MSP-EXP432P401R. Manivannan
Sadhasivam says it might've been overlooked, in
https://github.com/zephyrproject-rtos/zephyr/pull/21897

.

Signed-off-by: Ulf Magnusson <Ulf.Magnusson@nordicsemi.no>