pwm: microchip-core: Convert SOC_MICROCHIP_POLARFIRE to ARCH_MICROCHIP_POLARFIRE

As part of converting RISC-V SOC_FOO symbols to ARCH_FOO to match the
use of such symbols on other architectures, convert the Microchip FPGA
PWM driver to use the new symbol.

Signed-off-by: Conor Dooley <conor.dooley@microchip.com>
Acked-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>