regmap: fix regmap_read_poll_timeout warning about sandbox_timer_add_offset

When fixing sandbox test for regmap_read_poll_timeout(), the
sandbox_timer_add_offset was introduced but only defined in sandbox code
thus generating warnings when used out of sandbox :

include/regmap.h:289:2: note: in expansion of macro 'regmap_read_poll_timeout_test'
regmap_read_poll_timeout_test(map, addr, val, cond, sleep_us, \
  ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~
drivers/spi/meson_spifc.c:169:8: note: in expansion of macro 'regmap_read_poll_timeout'
ret = regmap_read_poll_timeout(spifc->regmap, REG_SLAVE, data,
        ^~~~~~~~~~~~~~~~~~~~~~~~
drivers/spi/meson_spifc.c: In function 'meson_spifc_txrx':
include/regmap.h:277:4: warning: implicit declaration of function 'sandbox_timer_add_offset' [-Wimplicit-function-declaration]

This fix adds a timer_test_add_offset() only defined in sandbox, and
renames the previous sandbox_timer_add_offset() to it.

Cc: Simon Glass <sjg@chromium.org>
Reported-by: Tom Rini <trini@konsulko.com>
Fixes: df9cf1cc08 ("test: dm: regmap: Fix the long test delay")
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Reviewed-by: Simon Glass <sjg@chromium.org>

arch/sandbox/include/asm/test.h

@@ -59,14 +59,6 @@ void sandbox_i2c_eeprom_set_test_mode(struct udevice *dev,
 
 void sandbox_i2c_eeprom_set_offset_len(struct udevice *dev, int offset_len);
 
-/*
- * sandbox_timer_add_offset()
- *
- * Allow tests to add to the time reported through lib/time.c functions
- * offset: number of milliseconds to advance the system time
- */
-void sandbox_timer_add_offset(unsigned long offset);
-
 /**
  * sandbox_i2c_rtc_set_offset() - set the time offset from system/base time
  *