esp32: select 8M clock as RTC_FAST_CLK on startup

esp32: select 8M clock as RTC_FAST_CLK on startup

Even though RTC_CLK_CONFIG_DEFAULT correctly had RTC_FAST_FREQ_8M as the
fast clock, the bootloader modified fast_freq field to the currently
selected RTC_FAST_CLK (so that the clock choice is not affected by the
bootloader). The application startup code never switched to 8M clock,
which caused the default (XTAL/4) to be used as RTC_FAST_CLK. This had
caused a number of issues, such as touch pads not working in deep sleep.

Fixes https://github.com/espressif/esp-idf/issues/542.
Ref TW12053.