earlyconsole prints question marks on 5.19.0-1002-generic

earlyconsole prints ������������������������ on 5.19.0-1002-generic

[ Impact ]

 * When adding earlycon to the kernel command line on the
   SiFive Unmatched board the serial console will print garbage
   halfway through booting until entering userspace and other
   other drivers are loaded.

 * The problem is the UART driver doesn't claim its clocks
   properly so the PWM driver will turn off the parent clock
   until PWM is needed for the LEDs on the board.

 * Compiling the PWM driver as a module so it will be loaded
   at the same time the LED drivers are loaded is a workaround
   until the following patch is backported
   643792048ee8 ("serial: sifive: enable clocks for UART when probed")

 * Only the SoCs on the SiFive Unleashed and Unmatched boards use
   this PWM driver, but none of them need it at boot time, so should
   be harmless.

[ Test Plan ]

 * Add earlycon and watch the serial console while booting.

 * Test that PWM leds still work after the PWM driver module is loaded.

[ Where problems could occur ]

 * There might be a hidden dependency on loading the PWM driver early.

[ Other Info ]

Original bug description:

With Kernel 5.19.0-1002-generic on the SiFive Unmatched earlyconsole output looks like:

EFI stub: Booting Linux Kernel...
EFI stub: Using DTB from configuration table
EFI stub: Loaded initrd from LINUX_EFI_INITRD_MEDIA_GUID device path
EFI stub: Exiting boot services...
�������������������������������������������

With a defconfig 5.19.6 kernel this does not occr.