Add support sleep for st7920, mini12864

[luxflow]

I saw powersave feature is implemented for st7920, mini12864 in u8g2
https://github.com/olikraus/u8g2/blob/30e78ca9890ac51df8cff4c99756d231eea21efb/csrc/u8x8_d_uc1701_mini12864.c#L70-L84
https://github.com/olikraus/u8g2/blob/30e78ca9890ac51df8cff4c99756d231eea21efb/csrc/u8x8_d_st7920.c#L62-L79

Is powersave feature in u8g2 different fromt sleep feature in u8glib?
if those are same, could you explain why it is not implemented in u8glib?
maybe?.. Note: There will be no more development for U8glib. Please use [u8g2](https://github.com/olikraus/u8g2/wiki).
Can I PR for this issue?

Thanks!

[olikraus]

if those are same, could you explain why it is not implemented in u8glib?

There is no reason. It is just like it is.

Can I PR for this issue?

to u8g2, yes.