arch/arm/src/stm32: Back out a small part of the previous commit ... my fault.

arch/arm/src/stm32/Kconfig

@@ -7365,7 +7365,6 @@ choice
 
 config RTC_LSECLOCK
 	bool "LSE clock"
-	select STM32_PWR
 	---help---
 		Drive the RTC with the LSE clock
 

arch/arm/src/stm32/stm32f30xxx_rcc.c

@@ -662,7 +662,10 @@ static void stm32_stdclockconfig(void)
 #if defined(CONFIG_RTC_LSECLOCK)
   /* Normally peripheral clocks are enabled later in bootup, but we need
    * clock on PWR *now* as without this setting registers that enable LSE
-   * won't work
+   * won't work.
+   *
+   * NOTE: In this configuration, we can assume the CONFIG_STM32_PWR has
+   * been selected.
    */
 
   regval  = getreg32(STM32_RCC_APB1ENR);