RISC-V: drop SOC_MICROCHIP_POLARFIRE for ARCH_MICROCHIP

RISC-V: drop SOC_MICROCHIP_POLARFIRE for ARCH_MICROCHIP

SOC_MICROCHIP_POLARFIRE doesn't have any remaining in-kernel users, so
time has come to remove it. Foolish auld me left the "POLARFIRE" in the
new, hidden, config option when I renamed it and changed all of the
users however. As HPSC has been announced, and is not a PolarFire,
drop mention of that specific FPGA family from the new user-visible
config option.

Acked-by: Palmer Dabbelt <palmer@rivosinc.com>
Signed-off-by: Conor Dooley <conor.dooley@microchip.com>