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We can have the following scenario:
1) Boot the board with at91bootstrap that has CONFIG_SCLK enabled.
 It will configure the external slow oscillator.
2) Reset board, or power down, replug.  Replace at91bootstrap with an
 at91bootstrap that has CONFIG_SCLK disabled.
 According to the at91boostrap help, this means use internal RC.
  Thus, at91boostrap will not touch this register (SLOW_CLOCK.SCKR)
3) Result is that the external slow clock crystal is still in use.
 Not touching SCKR , combined with battery-backed configuration means
 that the old config is still in effect.

This fix solves that.
CONFIG_SCLK is now part of a choice menu: either CONFIG_SCLK=y, or
CONFIG_SCLK_INTRC=y. When booting with CONFIG_SCLK=n, it will call the
function to go back to internal RC. When booting with CONFIG_SCLK=y it will
enable the external slow clock.
If the SoC has no slow clock whatsoever, there will be none of CONFIG_SCLK
and CONFIG_SCLK_INTRC thus no code is ca...