rtc: mc146818-lib: reduce RTC_UIP polling period

Waiting 1ms every time is not necessary, for example on some AMD boxes
the RTC_UIP bit is documented as being high for around 270 microseconds
in some cases [1], which agreed with experiments on an SB710
southbridge. So 100us seems optimal.

This in preparation for mach_get_cmos_time() refactoring.
The functions mc146818_get_time() and mach_get_cmos_time() in
arch/x86/kernel/rtc.c perform the same function and the code is
duplicated. mach_get_cmos_time() is busy waiting for the RTC_UIP
bit to clear, so make mc146818_get_time() more similar to it by reducing
the polling period.

[1] AMD SB700/710/750 Register Reference Guide, page 307,
https://developer.amd.com/wordpress/media/2012/10/43009_sb7xx_rrg_pub_1.00.pdf

        "SB700 A12: The UIP high pulse is 270 μS Typical when SS on SRC
        clock is OFF and 100μ min when SRC SS is ON." [sic]

Signed-off-by: Mateusz Jończyk <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Cc: Ingo Molnar <[email protected]>
Cc: Borislav Petkov <[email protected]>
Cc: Dave Hansen <[email protected]>
Cc: [email protected]
Cc: "H. Peter Anvin" <[email protected]>
Cc: Alessandro Zummo <[email protected]>
Cc: Alexandre Belloni <[email protected]>
Signed-off-by: Alexandre Belloni <[email protected]>
Link: https://lore.kernel.org/r/[email protected]

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