drivers: i2c: fix build break for nrfx_twi* with newlib

When running sanitycheck for reel_board which selects
CONFIG_NRFX_TWIM it fails the following tests:
tests/misc/test_build/test_newlib
tests/kernel/errno/kernel.common.errno.newlib
tests/lib/mem_alloc/libraries.libc.newlib

/zephyr/drivers/i2c/i2c_nrfx_twim.c:144:3: error: expected declaration
specifiers or '...' before '(' token
   (bitrate == I2C_BITRATE_STANDARD ? NRF_TWIM_FREQ_100K         \
   ^
/zephyr/drivers/i2c/i2c_nrfx_twim.c:151:3: note: in expansion of macro
'I2C_NRFX_TWIM_FREQUENCY'
   I2C_NRFX_TWIM_FREQUENCY(           \
   ^~~~~~~~~~~~~~~~~~~~~~~
/zephyr/drivers/i2c/i2c_nrfx_twim.c:184:1: note: in expansion of macro
'I2C_NRFX_TWIM_DEVICE'
 I2C_NRFX_TWIM_DEVICE(0);
 ^~~~~~~~~~~~~~~~~~~~
/zephyr/drivers/i2c/i2c_nrfx_twim.c:154:3: error: expected declaration
specifiers or '...' before string constant
   "Wrong I2C " #idx " frequency setting in dts");         \
   ^
/zephyr/drivers/i2c/i2c_nrfx_twim.c:184:1: note: in expansion of macro
'I2C_NRFX_TWIM_DEVICE'
 I2C_NRFX_TWIM_DEVICE(0);
 ^~~~~~~~~~~~~~~~~~~~
zephyr/drivers/i2c/CMakeFiles/drivers__i2c.dir/build.make:62: recipe
for target
'zephyr/drivers/i2c/CMakeFiles/drivers__i2c.dir/i2c_nrfx_twim.c.obj'
failed

To fix this, let's replace the use of static_assert() with a more
generic macro: BUILD_ASSERT_MSG()  This should work across minimal
libc, newlibc and c++.

Signed-off-by: Michael Scott <mike@foundries.io>
2 files changed