clk: microchip: mpfs: make the rtc's ahb clock critical

The onboard RTC's AHB bus clock must be kept running as the RTC will
stop & lose track of time if the AHB interface clock is disabled.

Fixes: 635e5e73 ("clk: microchip: Add driver for Microchip PolarFire SoC")
Signed-off-by: Conor Dooley <conor.dooley@microchip.com>
Signed-off-by: Claudiu Beznea <claudiu.beznea@microchip.com>
Link: https://lore.kernel.org/r/20220909123123.2699583-3-conor.dooley@microchip.com