arm: imx: Finish migration of CONFIG_CSF_SIZE to Kconfig

While in most cases CSF_SIZE is handled via Kconfig we have some i.MX8M
platforms that set the size based on the now-renamed CONFIG_SECURE_BOOT
symbol.  Update things so that CSF_SIZE itself depends on IMX_HAB being
enabled and provide the default value for i.MX8M family of parts.

Cc: Stefano Babic <sbabic@denx.de>
Cc: Fabio Estevam <festevam@gmail.com>
Cc: Ye Li <ye.li@nxp.com>
Cc: NXP i.MX U-Boot Team <uboot-imx@nxp.com>
Fixes: d714a75f ("imx: replace CONFIG_SECURE_BOOT with CONFIG_IMX_HAB")
Signed-off-by: Tom Rini <trini@konsulko.com>
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Changes in v2:
- Make CSF_SIZE depend on IMX_HAB and update to be default 0x2000 if
  ARCH_IMX8M after comments from Ye Li
- Reword the commit slightly