ATM2 keyboard microcontroller src code and etc.

Borrowed from a version made by Caro which in turn looks like disassembled from original binary image from honey soft.

Short-time objectives ('+' is already done):

• move completely to AT89S52 without supporting older KR1816VE31/51 chips, as well as dropping support for external ROM.
• only 11.0592 MHz quartz support
• add 115200 baudrate
• use extra indirect memory of AT89S52 for larger serial buffers/stack/etc.
• correct real-time clocks as they seem to lag because now timer is reinitialized to a constant after some timekeeping task