bcm53xx: fix kmod-i2c-bcm-iproc for kernel 5.10

bcm53xx: fix kmod-i2c-bcm-iproc for kernel 5.10

I2C_BCM_IPROC selects I2C_SLAVE which in turn makes two new symbols
available: I2C_SLAVE_EEPROM and I2C_SLAVE_TESTUNIT. That was causing:

I2C support (I2C) [M/n/y/?] m
  Enable compatibility bits for old user-space (I2C_COMPAT) [N/y/?] n
  I2C device interface (I2C_CHARDEV) [M/n/?] m
  I2C bus multiplexing support (I2C_MUX) [N/m/?] n
  Autoselect pertinent helper modules (I2C_HELPER_AUTO) [N/y/?] n
    SMBus-specific protocols (I2C_SMBUS) [N/m/?] n
  I2C/SMBus Test Stub (I2C_STUB) [N/m/?] n
  I2C slave support (I2C_SLAVE) [Y/?] y
    I2C eeprom slave driver (I2C_SLAVE_EEPROM) [N/m/?] n
    I2C eeprom testunit driver (I2C_SLAVE_TESTUNIT) [N/m/?] (NEW)

Ideally I2C_SLAVE should probably be packaged as kmod-i2c-slabe. For now
just modify kmod-i2c-bcm-iproc to fix kernel compilation.

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>